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.
the last 32bit only Mac system shipped a decade prior to Catalina. 32bit was dropped a decade after support for the last 32bit only Macs. Apple Silicon in both iOS and Mac is 64bit only, on iOS it has been for years.
Despite that, macOS supports everything required for proton - the mechanisms that allow wine to run 32bit x86 apps on both intel and under rosetta exist pretty much purely for the purpose of things like wine (and so proton).
So I'm not sure where you get "apple cut off the proton devs" from that work