I tried to see if this info was available in SteamDB (and it probably is somewhere — I’m just not finding it in an easily-accessible way), but my feeling is that most games on Steam are still Windows-only.
Your feeling is wrong. Here[1] are all the LSL games as supported on Linux via Proton, they are all playable (yes, even the red/borked one as that's actually Linux native). Most games are usually pretty well supported by Proton[2], for example all the big releases from Playstation games (God of War, Spideman, etc.) have had day 1 support as well as a lot of other big games Cyberpubnk 2077 had day 1 support as well. The main exception being multiplayer games with anticheat is the biggest roadblock right now. Though progress is being made with all the big AC engines (EAC, Battleye, etc.) working in Proton but needs to be enabled by each individual developer.
