Another solution is Conty[0] which is a download-and-run containerised Steam based on Arch; I use it to run games on Slackware without multilibs, and it's worked flawlessly so far.
However, it seems Arch are also dropping multilibs as a dependency of Wine, and moving to WoW64[1], with "reduced performance for 32-bit applications that use OpenGL directly".
What this implies for Steam on Arch (and hence for Conty) I'm not sure, though as of May some Proton versions have a PROTON_USE_WOW64 env var according to [2], so maybe multilibs can already be avoided running Steam natively anyhow.
