It is possible to define custom off-topic close reasons. The purpose of this thread is to collect proposals for such reasons (if there are any).
Please note that there is a strict upper limit on the number of such reasons we can have (three), and it seems like a bad idea to change them too often. Thus, I rather propose we exercise quite some restraint with introducing them, and do not fill up the entire list right away.
In addition, please recall that (as a beta site) we cannot (yet) have open migration paths.
Final technical remarks: the maximal length for the reason is 400 characters (the minimum is 25).
Some data (from the last 30 days, in the sense of what is shown in the relevant stats pages for this, which with autodelte slightly smears this over really 30 days):
We had 15 closed questions (8 visible and 7 deleted). Of these 5 to 6 seem like purely math questions (more or less suitable for math.SE but some not so good), 4 to 5 might be suitable for academia.SE (though rather not all); the rest a mix of genuine closures (too broad, unclear, and a like). In addition, I recall a quick selfdeletions of one question that was clearly a math question (that actually sparked that thread), and perhaps another one but this is less clear.
So depending on how one counts exactly, a third to a half are "math only" (potentially for math.SE) and a quater to a third are "not enough math related" (potentially for academia.SE).