On SO I've noticed a couple of "can't do's" recently, where I was prevented from doing something without a clear reason being given, and without a pointer to an explanation or a preferred "what to do instead".
One was tagging a question as "homework". Another was using the word "problem" in a question title. In both cases, I was shown a little red popup that told me that what I was trying to do was not allowed (and the prohibition was enforced). But there was no link or hint as to the reason.
Now I see that when you use "problem" in a question title, you actually do get a link to an article about the reason for the prohibition. So that feature request is already implemented. Great! (I do think there are cases where working around this broad rule results in a worse question title, but I see the value in avoiding redundancy for the majority of questions with this word in them. And I'm not inclined to join a big argument over the rule, now that I understand the rationale.)
Similarly, if you try to create a tag that's too similar to an existing tag, you get a red pop-up with an explanation, as well as being prevented from creating the new tag. That's helpful!
I found the reason for the removal of the "homework" tag on meta; and the "what to do" is apparently that you may mention that your question is related to homework, just don't tag it that way, because it invites comments that beat a dead horse and contribute nothing. That's a legitimate reason. But in the absence of that explanation, the prohibition is bewildering to someone who had some passing acquaintance with the old homework tag, and who knows about the FAQ but doesn't know to search meta. The FAQ doesn't say anything about homework, that I could find.
I'm not looking to reopen the big debate about what to do with the homework tag. I'm only making a feature request that prohibitions should give a link to some sort of explanation and a suggestion of what to do instead (e.g. what mental model to use that's more compatible with the community's direction). That would make the difference between a user experience of "I can't do what I think I'm supposed to do, and I don't know why" and "I can't do what I thought I was supposed to do, but now I see that I'm supposed to do Y instead."