Valve seems to be testing ARM64 and Android support on Steam, as per a new SteamDB leak. Popular titles like Left 4 Dead 2 and Garry's Mod are being tested with "proton-arm64" tags, among others. This could hint towards new future Valve hardware running on ARM64 architecture.
If anything I’m betting this is for their rumored standalone vr headset they have in development as a Quest competitor