Just my 2¢: occasionally questions that are "unanswerable" in terms of their topic matter are actually answerable in the SE format. In the case of a well scoped questions asking about the answer to some riddle, a valid answer that only an expert may be able to give could say "we don't have enough information to know". That in itself might be the definitive expert answer that is needed and closing such questions doesn't serve any purpose over answering. Of course [the more common] questions that insist on attempts to speculate in spite of that awareness can be closed forthwith as opinion based. — Caleb ♦ 7 mins ago