This ended up quite long. Here's the short version:
A lot of comments are noise. We don't expect people who are trying to use the site, or help manage the site with their tool-access powers from high rep, to wade through a swamp of comments to understand what's going on. We doubly don't expect that on a question that's been abandoned by the author. We triply don't expect that when the comments move into argument territory.
When comments are noisy and the post they're on is a “current event”, we remove any that are unnecessary. What comments are necessary depends on context. The context here was 1) the question was closed, 2) the author had abandoned it, 3) there were reopen voters who commented some really wrong things about what's on topic at RPG.se. Comments were all removed, except the comments that served to address (3), or which were informative to maintenance-privileged users about why (1) and (2) were the case. (2) also meant that there was no point in preserving any comments that were part of dispute or discussion about the closure, beyond what was necessary to explain (1) to users who arrived late to the party.
No comments were left that didn't need to be there to serve those needs.
Trying to childishly manipulate the perception of an argument that was no longer even visible was not a motive in any of the decisions to remove or keep a comment. Rest assured, the comments were handled in good faith and adult fashion.
Part of the difficulty with these comments appears to be that several unrelated subjects were all mixed together at once (closure, topicality, sibling sites, voting habits, the nature of comments, the nature of meta, author intentions — to name a few), making it very easy for what any given comment meant to be misunderstood and the responses to deepen the misunderstanding. This is exactly why comment chains that get messy like this are nuked and the participants redirected to start over in a fresh meta question, where there is room to untangle and avoid messes.
Another problem is that the question is clearly off topic according to our precedents. If there is nuance to the question that means our precedents need to be be re-examined, that's something to happen on meta, not in comments. (Note that though we are now on meta, this meta question about comment handling is also not the place to tackle re-examining precedents. If that is desired, a meta on that subject should be asked.)
That's the short version. Read on for the long version.
Foreword: Why precedent makes it clearly off topic
Let's dispense with this quickly. There were definitely misunderstandings in the comments about this point, and I believe that contributed to the volume of comments and well as many misunderstandings and disagreements.
The question is, on its face, clearly off topic. It is a question asking for a vague list, and it's asking for information that is not the domain of RPG experts. It doesn't matter if RPG experts would answer it better than Mythology.SE experts, as one comment contended. The fact remains that we are experts in RPGs, not in South European folklore. We don't have a folklore SE, but the existence of a more fitting SE has never been relevant to whether a question is topical, only the existence of more relevant experts.
Given that it was a real-world folklore question and asking for a vague list, there was nothing to dispute — it's not a question about roleplaying games. Perhaps this is something wrong with our precedents — in which case, a meta about that should be started.
The only thing happening was a fight (based on a misunderstanding) that the mods were called in to break up.
Preface: Behind the curtain regarding comment pruning
When comments are removed, we always weigh the effect that the removal will have on several possible audiences, including: the author of the post; the author of the comment(s); the readers who saw the comment(s); the readers who will never see the comments; any voters; any answerers; and any high-rep users who have taken moderation-tool actions on the post such as edits, close votes, or reopen votes; any high-rep users who haven't yet taken such actions.
The ideal comment-handling choices for each of those groups don't fully overlap, so every time we consider removing comments we necessarily have to make some compromises and try to decide what is the most productive for the question itself and the site as a whole. We also have to make the choices in a reasonable amount of time, both in order to have any useful effect on a comment chain that might need action, and in order to move on to other tasks waiting for us. We get a lot of practice, and are pretty good at making these choices quickly and in a principled way, while knowing perfect action is unachievable.
We do this in good faith, and with an eye for what is best for the site. In case that sentence provokes an objection that we shouldn't be deciding what's best for the site, allow me to ground that a bit in the utter pragmatism that moderation demands: when we say “best for the site”, we have in mind our mandate to prevent the site from going down in flames, not our personal idea of a utopian RPG site. When we are considering what's best for the site, we're mostly working to prevent or interrupt things that are not best for the site: flame wars, edit wars, comment arguments, using the site for discussion, insults, provocation, stalking, serial downvoting, not using answers posts for answers, not using question posts for questions, “fun” off topic questions not being closed, difficult question being closed that shouldn't be, sockpuppeting, trolling, spamming, … and I'm going to stop myself there, because that list could go on for a few more paragraphs.
So in context, when we remove comments, we are always acting to prevent or end a problem that is occurring or about to. “We're right, you're wrong” is so far down the list of motivations that we can't see it from here. We're too busy with more important considerations to get around to personal ones like that.
We take care of those janitorial, maintenance, and site management issues so that y'all don't have to (but with enough rep, can), and can focus on the site's content in a way that moderators aren't able to. Y'all have the ability to get personally invested in a way we don't, because you don't have to first think about site management (but can if you want) before you can think about how many arms a PC needs to be able to use sixteen zweihanders.
And in case it needs saying again: comments are by-design temporary. Removal is the default, and leaving any given comment un-removed generally serves a site management purpose. (Though sometimes that purpose is merely “there is something more important to deal with right now”.)
So, that's the very long version of where we're coming from when we're looking at a bunch of comments.
In this case
What happened is that a lot of comments were written. This automatically flags the question for moderator attention, because the purpose of posts and comments on Main is such that there should never be that many comments on a post long-term, and therefore it's time to do some weeding to get rid of the unnecessary, unconstructive, obsolete, or inappropriate comments in order that the useful ones (if any) remain.
This is my evaluation of the first set of removed comments in roughly chronological order, and my explanation for their removal:
Some discussion and suggestions about other places to take the question. The author interacted with these, therefore the information had been received. These didn't need immediate removal, but would eventually be removed as no longer needed.
A user relatively new to their reopen vote ability expressed some rather wrong ideas about how the site works. Perhaps their general position about the question's topicality was correct, but their reasons weren't. These were removed because disinformation doesn't serve the site, even if it's well-meant. They were also removed because they were obsolete once the commentors were asked to take it to meta instead. They were also obsolete when the question was abandoned. So they could have been removed three times, but we can only remove a comment once.
Comments debating whether the question merits closing, or is okay to have on RPG.se. Those that were removed for being obsolete. I'll comment on this more below.
Discussion of the voting on the answers. Entirely out of place in the comments on the question and easily removed.
Many comments, including those mentioned already just above, were arguing unproductively. Tempers appeared to be heating. In such cases, comment removal is like digging a firebreak to contain the spread of a forest fire — remove the things that are making people angry and that are inspiring new comments, and the argument will disappear unless it's a very hot fire. (We have other ways of dealing with fires that refuse to be contained by modest means like removing comments.)
The comments debating the close
There were some comments debating the close. All were removed except one, and I want to address why that is, since it seems to be part of the objection in this meta question.
Recall the various groups who are affected by comment removal decisions. Who is the most important group to consider? In this case, it's those users with moderation tool access who have acted on the question and those who haven't yet seen the question, and those users who were involved in the fight. The first need context for what's going on with the question, and need to see the explanation offered for the Too Broad closure, so that they can evaluate it for themselves and vote accordingly. The second group need to not have new inbox notifications to respond to. This entirely explains the comments about the debate that were removed, and the ones that were left.
- One comment giving context for the closure was left, as above. (This is the only comment that was part of the debate.)
- Two were left pointing to relevant meta discussions. These help spread awareness of how the site works for users who are new to their close/reopen vote privilege, which was an issue in this case. Leaving these is useful to improve how our collective site maintenance works. (They were in the middle of the debate, but didn't take part in it — they merely provided a neutral resource that disputants could consult and refer to.)
- One comment that came after the dispute was over was left existing. This was a moderator's comment that did two things: correct a misapprehension about list questions that the author restated in response to the second link (perhaps the author missed the first link in the flurry of comments), and noted for the sake of others that the author had abandoned the question. (This comment overlaps the two sets of comments. See below for more.)
- Comments that were heating things up instead of calming things down were removed, though mostly this was redundant with the aim of having minimal comments.
Notably, the moderator wasn't involved in the dispute. From a moderation perspective, there was no dispute to get involved in, just a number of users who were correctly applying established precedent on a fairly clear case of an off topic question, who were being given a hard time by users who didn't appear to be familiar with these established precedents.
This isn't picking sides, unless you count “the site” as a side a moderator can pick. This is a moderator breaking up a fight and dousing the flames by removing the comments that were fueling them, while retaining just enough to keep high-rep voters informed about what's up with the closure.
The second set of comments
That's the first set of comments. Fairly tame, nothing too unusual there. The second set of comments is where things seem to have gone off the rails.
These comments started with a pair of comments from a mod addressing the significant misunderstandings that were in the first set of comments.
- A comment explaining why the question was closed. Previous comments about the closure were largely missing the point, and this comment aimed to remind everyone of the fairly clear reasons available to close the question: it was asking for a list, and it was a campaign research question that did not ask anything that needed RPG expertise.
- A comment explaining that it's not unusual for voters to downvote answers to questions that are obviously off topic. This wasn't to justify, so much as point out that it happens: it's a thing that voters do, and up/down voters can do exactly as they please (apart from serial voting), and objecting is needless use of yet more comments. Explaining that this happens was in response to several previous comments on the subject.
What followed were mostly increasingly heated comments asking what the point of that mod's comments were, and (ironically) strenuous objections that the two comments artificially continued the debate. These comments were largely unconstructive. The moderator's comments addressed a number of misunderstandings, but the number of new misunderstandings that arrived with further comments indicate, to me reading them after the fact, that there were too many misunderstandings to fruitfully deal with in comments. I am unsurprised that the moderator cut bait and deleted the lot of them.
Notably, one of the two comments that began the second set of comments was removed: the one that explained why it was closed and likely to remain so was left to serve as context for other users with site maintenance on the mind. The second comment addressing the misunderstanding about how people vote on answers was removed, likely because that whole tangent had proven fruitless to address in comments. The rest were removed, because comments aren't for angry arguments.
Allow me to address the alternatives you propose:
- Removing the comments wholesale.
This would have left no context for our community members with access to moderation tools, so would have been inappropriate for a freshly-held question that had been the subject of dispute.
- Remove all but the last line: "[There was discussion, but] the author has indicated they don't want to do anything more on this question so it'll probably stay closed." (To indicate that the fact that the question was closed need not be further discussed.)
The first half was addressing a point being missed by the author (that we don't do list questions, and it clearly was one). This is important, because it's material context for why its closed and likely to remain so in its current form. I addressed this in more detail above. The second half was addressing the general fact that it's likely to remain closed, because the author indicated they would be allowing the question to remain in its current obviously off-topic form. These two halves only really made sense together.
- Moving the whole discussion to chat.
Productive discussions do get moved to chat, but arguments never do. That wouldn't serve our function as fire-fighters, as it wouldn't remove the fuel from the fire. We're not here to enable arguments, we're here to prevent them.
- Moving the whole discussion to meta.
There's no such moderator power. Since there's no site mechanism for converting comments into posts anyway, such a power is unlikely to ever be added.
Instead, we ask that people having a dispute take it to meta. That was done twice by a moderator. Since the suggestion wasn't followed, that was the extend of our powers to move a dispute to meta.
I guess we could have also locked the post against further comments with the “no really, use meta for that” reason, but that seemed unnecessary here. It's often too strong an action to merely repeat the request to use meta for its purpose instead of misusing comments.
Other things of note
“Which is just inviting serial downvoting”
The term “serial downvoting” seems to have been misused here. Serial downvoting is when someone goes to the user page of a particular other user to find the list of their posts and starts casting downvotes on all their posts. That's an abuse of the system and results in reversal of the votes and possibly a temporary suspension for the abusing account.
The meaning that seems to have been intended is that “Which is just inviting more downvoting”. Perhaps it will or perhaps it won't. How comments might affect the choices of our voting public is pretty much irrelevant when we're considering whether it's time for a comment to be removed — there are far more important considerations that leave no room for such a minor issue as how others' votes on the question might change based on the visible comments, which isn't even our business anyway.
“quite detrimental to discussion culture”
Excellent. RPG.se doesn't have, and is designed to kill, discussion culture. As wonderful as a healthy discussion culture is on a discussion site, on RPG.se it's a sign of disease — much like a healthy human gut culture is wonderful when it's in a human's guts, but in another part of the body is a sign of disease. If what we're doing is successfully killing any growths of discussion culture, we're on the right path.
“deleting a question with answers to it can lead to a question ban” (from one of the removed comments)
That's not how question bans work. Someone has to have a really terrible question record to earn a question ban.
The exact formula for the bans is not disclosed, but users are only banned if they have a significant number of heavily down-voted, zero-voted, or deleted posts.