Even gamers nexus' Steve today said that they're about to start doing Linux games performance testing soon. It's happening, y'all, the year of the Linux desktop is upon us. ᕕ(ᐛ)ᕗ
Edit: just wanted to clarify that Steve from GN didn't precisely say they're starting to test soon, he said they will start WHEN the steam OS releases and is adopted. Sorry about that.
you don't have to onsider off platform titles on its own. just take proton DBs list and sort by playercount and youll have your handful of misses on some of the top currently played titles. that already filters the non steam games already, and it still has its small handful of titles not on board yet.