On this site, we routinely get questions that are about one specific site. Oftentimes, it happens that users search for things related to their meta question, and end up discovering our site.
After having looked at the help center page and the sidebar shown to users, nowhere do they say that site-specific questions are in fact off-topic. They effectively boil down to "your question should be about Stack Exchange", and questions specific to a particular site are indeed technically about Stack Exchange. Additionally, there are a lot of old questions from back when this site accepted questions specific to Stack Overflow, many of which haven't yet been closed.
Since nowhere in the guidance does it say otherwise, and given the large number of not-yet-closed SO-specific questions, I think it's safe to say that users aren't made aware of this policy until they actually ask their question and it gets closed. I've been leaving a canned comment on such questions, and in the majority of cases the authors have honestly apologized and moved their own question to the correct per-site meta.
The answer below argues against this, because they think this would be creating a new, fixed rule, and that many users are aware of the per-site meta and choose to post here anyway. However, from what I've seen, the vast majority of users who post site-specific questions here are honest users simply asking about one of their posts or suggestions (and not venting criticism against a site's population), and once I politely tell them that it's off-topic here, they then apologize and go on their way. Additionally, it's just wrong to not indicate that something's off-topic in the help center if we routinely close questions about them and have a dedicated close reason for them.
Can we please add an explicit statement to the help center page and the sidebar that "questions about a situation on one specific site should be asked on the relevant per-site meta instead"?