This is a post notice. It's like the banners mods can put under questions, except this one is for any post, including answers.
The “notice added” event will show up in the post's revision history; this one just doesn't have an easy link to its revision history yet because of no edits done so far, but you can manually cobble together the link to see its revision history. On answers with edits it will be easier to see when/how it was added, so it's not meant to be a stealth action.
Why is it there?
The post notice serves to indicate that citations are needed, and the post lacks them. The implication is that it needs revision to say something of substance in response to a rules-based question; in particular, it needs to back up its assertions with experience or citations to experience.
It also serves as notice and warning that it may be deleted in the future, failing revision. That last is might be the biggest job it's doing there.
Who put it there?
I did! It's step 2 after mxyzplk's comment saying “Personal opinions aren't an answer to this question.” got no response.
This is a relatively new tool (as in, sometime in the last year?) that was added quietly and we've only discovered recently.
Is a post that outright states "this is how I would rule" really on notice for deletion?
Like bellybuttons, everyone has an opinion; that's why we close questions that are primarily opinion-based, because there are an endless number of opinions that can (and will) be submitted on them. We don't want endless opinions on non-opinion-based questions, like that one, either though.
Opinion-based answers that contain no justifications (citations to personal experience with it working) serve as broken windows that make it seem like it's OK for other new and experienced users to just throw in “my 2¢” of opinion. The post notice also serves to say that answers like this aren't an example to follow.
I've never seen this notice anywhere else--should I be flagging speculative answers to receive this notice?
If an answer doesn't Back It Up! or follow GS/BS and it really should, sure, send a flag our way. We might not agree on every flag, but an equilibrium of understanding in how the notice gets / should be applied will eventually develop.
Do we really want to be tagging speculative answers with  stickers?
Yep. This answer almost didn't get one, because we don't want to go overboard with this. Often, downvotes can take care of borderline cases. But in this case it's warranted. The answer starts by saying that it's not a rules based answer — which might be a fine introduction to a frame challenge — but then finishes up by saying that the reason it's not backed up is because they've never used these speculations:
This is not a rule-based answer
I have never had to use the above concepts or rules because [I have never needed to.]
That's a bellybutton answer. Everyone's got one, but thankfully most people refrain from sharing when unasked. When someone does share, we don't want to encourage it, and often we're going to remove them.