I'm glad you asked!
That is post notice. It is something that community (diamond) moderators now have an option to add to questions and answers.
There are three of them:
- Citation needed (the one you quoted)
- Current Event (which I think doesn't really apply to this site. Most things that could be marked with this are probably OT)
- Insufficient Explanation:
We're looking for long answers that provide some explanation and context. Don't just give a one-line answer; explain why your answer is right, ideally with citations. Answers that don't include explanations may be removed.
(Which is sadly only applicable to answers, due to its wording).
There is also a "lock" status that can be put on older questions that comes with this post notice:
This question exists because it has historical significance, but it is not considered a good, on-topic question for this site, so please do not use it as evidence that you can ask similar questions here. This question and its answers are frozen and cannot be changed. More info: help center.
I would much rather see older questions flagged for this kind of lock, rather than see them get closed years after their post date.
Either we just recently got the option to deploy these, or it's been there a while and I didn't notice until about a month ago due to my own incompetence (this one is probably more likely). But either way, they are an option now. I'm not sure if there's a way on this site for users or reviewers to get these added. However, if you see a post that could use one of these, flag it with a note saying so. Also please flag to get them removed, if one is no longer applicable, of course.