I was trying to find the original justification for why macOS was removed from Proton, but could not find any official word apart from some comments here https://github.com/ValveSoftware/Proton/issues/1344.

It was quite odd because the original herald of Steam Play had macOS front and center, even with exclusive cosmetics if you launched the game on macOS.

I do remember a large kerfuffle over the very poor state of OpenGL on macOS, which if I remember correctly was stuck on 3.2 for a long while, before being completely deprecated.