A while ago, I asked this question on ancient Arabic culture and how it relates to mythology, and it seems to have been well-received. At least it wasn't closed.
More in general, however, I could see questions on mythology being turned into philological questions. If I were to ask whether Freyja was Odin's wife, that would, of course, be an off-topic question of mythology (and probably a controversial one at that), but what if I asked what historical attestations there are on Freyja being Odin's wife? In a way, I think this is a reasonable question on historical literary sources and historical culture, but on the other hand I could see questions that are about mythology at their core being turned into such questions.
EDIT: More generally, I guess I'm a bit confused about the help center listing "cultures" and "ancient languages" as on-topic but "mythology" as off-topic, while these topics seem to have an extreme degree of overlap. Where to draw the line?