Look if people are getting mad because you're using "they" to refer to someone who has not disclosed their pronouns, then they're being unreasonable. But I've literally never seen that. What I have seen is people getting mad when someone uses "they" to refer to someone who has disclosed their pronouns. And often, this is something that is done by mildly transphobic people who aren't fully comfortable using, e.g., "she" to refer to someone who they feel looks insufficiently like a woman, but recognize that using "he" is wrong. Using "they" in this situation is just as wrong.
That being said, it doesn't help that there's no easy way to disclose your pronouns on this app. I'm a lot of discord servers I'm in, they use badges for pronouns so you can click on someone's username and see their pronouns. But no such option exists here