Probably a hot take but I disagree, it's the same issue as the whole "trap is a slur" situation that I also disagree with.
The issue is that, if the word is used against a trans woman without bottom surgery, it's a term that might not fit or be appropriate. With trap, it's just straight up misgendering, but it might fit someone who still identifies as male. With futa, (if I underatand your point) it might carry a lot of sexual meaning with it which could be inappropriate.
It's an issue with "who uses the word to describe who", and therefore isn't in my eyes a slur.
Edit: clarification
Wait tr*nny is a slur...? Why?