I don't think there should be a rule either way, but instead a community guideline to avoid pronouns in general if they're not known, or to use they/them if absolutely necessary.

The only rule related to this, imo, should be civility. If you're intentionally misusing pronouns, that should be against the rules, not because of pronouns, but because you're intentionally being a jerk. The same goes for slurs, personal attacks, etc.

It turns out that I've seen a lot of stupid comments from users from hexbear, but that's potentially because those users are really easy to spot due to the unique username format. Regardless, I think it's a stupid instance rule, and I reflexively skip past those posts because I haven't seen any good posts (subjective obviously) from a user from that instance.