If you don't do anything weird, you land in that 80% and everything works as it should. With developers noticing SteamOS being a thing, more of them start doing sanity checks to make sure it works on Linux, and that 80% starts growing to 90%.
Then there's the kernel anti-cheat that's unfixable though, which pulls the percentage down again.
Of the top 1000 games it seems 77% are playable. 40% of it needing "some tinkering" but I dont know what that means