I’ve known people to remove nipples as part of extreme body modification but not as gender thing.
In my world, this stuff is done by body piercers and not doctors. This is a legal grey area (can you consent to harm?) but I would imagine it’s far more ethically troublesome for doctors, I mean, what diagnosis would you need to justify it? It can’t be considered purely cosmetic, surely? I mean, you can justify a nose job based on aesthetics/self esteem, but you wouldn’t remove a nose entirely based on the same argument...