I encounter a few native Linux games from time to time on Steam, mostly indies and other smaller games. There was a time there was a medium-sized push for this, Steam was pushing it, lots of Humble and GOG stuff came out with Linux versions...

But yeah, Proton is so good now that I don't think there's much impetus to port. Test on Deck/Steam Machine/Proton, sure, but not so much port. Steam Play also handles runtime container stuff for native Linux games so that can be pretty good and stable itself, but I've definitely had situations where switching over to the Windows version via Proton is a better result than the native Linux one.