I remember using bare wine to play games before proton. You would have to go and find the exact libraries needed to run the game, install them one way or another, pray a bit, and maybe the game will run with acceptable fps. If it ran at all.
And these days its just plug and play. Dont remember the last time I had to install a game dependency with proton, from steam or otherwise.
I was wondering that too. As far as I know, when it comes to Linux, AMD and Intel are the way to go. Nvidia are the ones who generally tend to suck on linux (although I never had problems with my nvidia gpu, its pretty old tho)