I'm probably a bit further to the right than most on the fediverse with this opinion but...
I think, once you have been informed of someone's pronouns, it's flat out rude to not use them. I don't know if it's a banning issue but that's for the moderators on your instance to decide or the instance the community is on. Even if you don't agree with someone's lifestyle, it's just polite to address people the way they'd like to be addressed.
But surely there's a difference between intentional misuse and accidental. I think banning someone for not looking up someone's pronouns before a public interaction seems like pushing things a bit far here. I certainly am not checking such things. But, then in general when online I will use gender neutral wording because frankly, for online interactions someone's rarely information that matters for the interaction. I don't really need to know.
My view is, I think it is almost always clear when someone is being malicious and thus transphobic and when someone makes an honest mistake/did not know better. We, as a whole, really should be differentiating between obviously malicious and non-malicious cases.