I saw a little chatter about this in previous questions, but I just want to be totally sure: In cases where a character's assigned sex at birth never factors into canon (or, to put it a different way, where it's never confirmed whether a character is canonically cis or trans), is it cool to play them as transgender?

For a random example, Auron from Final Fantasy X's narrative has very little to do with gendered social pressures involved in being assigned male or female at birth and we also don't see his bare chest or junk at any point. Would it be acceptable to play him as a trans man, or would that be considered an AU here?