My opinion on this might be in the minority, but I am going to post this anyway, as a suggestion. If this is inconsistent with policy, consider it a suggestion for a change of policy. As a premise to my opinion, I note that there is a general consensus that "textbook problems" are valuable problems for statistical learning, and so we do not close or ignore these problems. Our present practice for self-study questions is to require the user to show what they have done, and where they are stuck, and then provide a "hint" for further progress.
I agree that it is sensible not to provide contemporaneous solutions to self-study problems, since this gives the user a facility to have others do their homework for them, and it encourages students to post homework or assessment problems. However, setting aside the issue of contemporaneous answers, I see no particular problem with this site gradually accruing a large repository of full worked solutions to textbook problems. That actually seems to me like a valuable resource, for exactly the same reason that worked solutions in textbooks and courses are valuable resources for learning. As I see it, the issue is to avoid giving contemporaneous worked solutions to the student for their homework/assessment. Thus, once the question is "old" (e.g., at least six-months old, which is past the assessment point) I think it should be okay to post a full worked solution to the problem. Indeed, at that point, I think it is desirable to give a full worked solution, to build up a repository of worked solutions to simple problems.
This proposal retains the property that it does not incentivise students to post homework or assessment problems, to bypass solving these themselves. By imposing a delay of six months on posting worked solutions, we ensure that the full solution comes too late to be used as submitted homework or assessment in a course. (I actually think this issue is a bit overblown anyway, since students almost always need to replicate performance on an in-person exam, and SE solutions cannot be used as a substitute in that environment.) At the same time, it allows us to build up a good repository of worked solutions to simple "textbook problems" which is a useful pedagogical resource.