The texts are now fine for me with Proton 4.1-2, including proper subpixel rendering.
I don't know more about the details, but i guess this issue was fixed some time ago in wine-staging (since I had no problems when using staging), and now made it's way into wine / Proton.
proton 4.1-2x1 2019-03
In Wine programs (like winecfg, regedit, explorer, ...) there is no text anti-aliasing at all with Proton. This seems to cause ugly fonts in some games, too (I guess). I installed "allfonts" and tried various Wine settings (with winetricks and regedit), but this didn't change anything.
Semi-related question: Is it known, why Steam itself "only" has grayscale anti-aliasing? Is there any way to change this to subpixel rendering?
For normal Wine to have text anti-aliasing, my system must have subpixel rendering activated (not grayscale!). I don't know, though, if there is any correlation between Steam and Proton font rendering.
System information: https://gist.github.com/zzeiSt/2f3143930b594adf9bdd7b75419d3871