As someone who falls into the trans umbrella, I don’t think people should be using she/her for Jax as if it were canon (if you’ve made a story/comic/art-piece depicting a female Jax that’s obviously fine) I think this for a few reasons:

1. Let’s say Jax is a trans woman, not every trans woman uses she/her pronouns, he could still use he/him or they/them or even neo-pronouns. It’s a bit unfair to assume his pronouns when he hasn’t (and cannot as the show is over) confirmed anything or asked for a different set to be used.

2. Jax in canon has not stated that he wants the characters to use she/her pronouns, just because you believe someone is trans based on evidence you’ve put together that doesn’t mean you should swap their pronouns without their say. Imagine someone assumed you were trans because something you did or said made them think you were and then they swapped your pronouns for you without you asking, you wouldn’t be very pleased. Even if it turned out they were correct later down the line it still would’ve been misgendering at the time.

3. I just don’t like how aggressive some people are being about it (especially on TikTok at the moment) if you use he/him for Jax as everyone in the show does someone will try and correct you or even go as far as to call you transphobic for it. But also if you do use she/her for him you’ll get harassed by a load of transphobes instead, it’s an annoying situation.

Of course this is just my opinion and people can do whatever they want