A mod said I should ask if there is a rule that says one single (arbitrarily defined) question per post. Too Broad does not mention it. Is it an actual rule somewhere or a guideline or what? And how does it define 1 question? Questions can need very broad answers (like explaining how a spell works in many cases) or a single yes or no. Is it OK to ask a few yes/no questions, but not many broader questions (like for example how does magic jar work against ethereal creatures / how does magic jar work against constructs). If a complete answer can be given in 10 lines of text, is it still too broad a question?
A hard and fast "follow this to the letter" rule is unnecessary and counterproductive. Instead we apply principles of site design which we've learnt through years of experience are the best way to curate the quality of the Stack.
I suggest you read Optimizing for Pearls, Not Sand to understand why we're tougher on questions than on answers. I found it crucial background material for understanding the Stack's curation policies.
Our own @thedarkwanderer has written a pretty good summation of the principles in play: experience has shown that questions which can be answered separately should be asked separately because it encourages more, better answers and makes it easier to find them.
There's no invocation of length or complexity when dealing with this principle. It's about whether the questions can stand on their own, that's all. This helps the Stack be a well-sorted pile of solutions to problems, by letting us sort each post on its own whenever reasonable.
And if you see an old question which seems to be two or more mashed up unnecessarily? We're fallible. Flag it for attention, it's probably slipped through the cracks or dates from an era when we didn't understand how important this is. The Stack doesn't take precedent very seriously; we're constantly updating old posts to fit our new understandings of how to best serve the site.
It's well established SE-wide best practice to ask one question per post. You can read this post and its linked posts on meta.SE for all kinds of why.
Therefore, we tend to vote multi-question questions as too broad to prevent all kinds of other work later when it turns into a mess of answers addressing N of M of the questions, and then later needing editing or splitting once someone wises up.
It serves both the OP better, and other users on the site and search, because they can more easily find specific answers to questions than muddled overlapping explorations of whole topics.
(Reposted by request from Why'd my second question get removed when surely people would be thinking about both?)
The biggest benefits here have little to do with questions. It's about the answers.
1) When I post an answer to a 'question' that's actually a bunch of related questions, I have to address them all (at least, if I want to be a good site member); that's more work for me, not always possible to format smoothly, particularly if your 'related' questions turn out not to be related after all, and especially when they're only related by a 'people are probably thinking about this now, right?' kind of logic train. This makes your question harder to answer, but it's not because your question is uniquely challenging, it's because your question sucks (warning: TV tropes links. The analogy is, in fact, illustrative, though, so I suggest you read the article intros at least). This is bad.
2) When I post an excellent answer to a question, I am rewarded by upvotes. When I answer any number of related questions posted as a single question, I still get one upvote. That is bad.
3) When I am reading an answer, I have three primary voting options: upvote, downvote, or don't vote. When an answer actually is a single answer, deciding what to do is easy; I upvote it if it's useful/right/etc, downvote it if it's bad/wrong/etc, and don't vote if I'm not expert enough to tell the difference/need clarification/etc. If your question forces answers to have multiple parts, it is normally the case that I find myself wanting to upvote an answer for one part, but wanting to downvote/abstain due to another. This is one of the biggest cited reasons not to do the 'related question' thing, and, while the same thing can happen even on regular questions, substantially increasing the risk of putting voters through this is bad.
4) When I post questions like this (We higher-rep people do things wrong, sometimes, too), I usually find (at least) one of my questions ignored, in favor of another one, as in this case (the top answer is just straight up not an answer to the question at all; ignore that one. The rest are good examples, though). This is frustrating, and while it usually feels like the answerers are the ones doing things wrong, the blame shouldn't be entirely on them; I should have posted the question as separate real questions to start off with. Not doing so is bad.
5) Answers are supposed to stand on their own. When I answer a question like this, I may find myself needing to repeat an entire other answer in whole or long-part, because it is the right answer to most of the question. This makes my answer more of a comment on a part of that answer and there is no real good way of dealing with that when my answer is completely identical for entire questions. Leaving a comment might work, but it doesn't if the answerer is not persuaded by my arguments, and I shouldn't be using comment-arguments to persuade an answerer to change their mind anyways, ideally.
So, basically: No, we should not change the policy here
P.S. We are the mods and we make the policy. I mean, there are elected mods, too, but even more than SE in general, this site is really and truly community moderated. SSD is an elected mod, but the editing of your question was well within the purview of regular normal user activity. Not trying to call you out or anything, just a heads up, in case you hadn't realized how we do moderation around here.
Separately, deleting a question because of this is excessive and silly, and that probably shouldn't be happening, so I'm with you on that one. Locking makes sense in a content dispute, and deleting makes sense for old questions that we have reason to believe are abandoned, but it's very scary-looking and not something we should be doing without serious consideration, even if it is easy to reverse. But I, at least, haven't ever seen a question deleted because of this without signs of abandonment.