John Rennie's answer in the meta thread currently has ~3x the upvotes of ACuriousMind's accepted answer. This seems to indicate we should allow check-my-work questions.
Why do we still consider these questions off-topic?
The way I look at a question is to ask whether the answer is going to be of interest to many members of the site or just to the one person asking the question. For example I've answered many questions on special relativity that are dangerously close to check-my-work because I felt I could explain a general approach that would help not only the OP but lots of other people trying to learn SR.
On the other other hand I've close voted many more check-my-work questions than I've answered because I felt the answer would achieve nothing but to get the OP a better grade on that week's assignments.
The problem is that check-my-work is a vaguely defined category that is not an especially useful way of classifying questions, and therefore your question is not a well defined one. The point of the answer of mine that you linked is that we should look hard at check-my-work questions and decide whether they are really asking for an explanation that is (a) conceptual and (b) will be of interest to lots of people.
A blatant "spot my deliberate mistake" question is always going to be off topic, but I would urge site members to consider whether the mistake is conceptual rather than arithmetical. If it's a conceptual mistake I think there is some justification for answering the question as long as your answer is mainly concerned with the concepts involved.
This isn't "check-my-work questions should be allowed." This is "please be discerning about what is a check-my-work question and what is actually a conceptual question."