I would hope they'll support creating win32 prefix, some games may need that. I was never able to get Sonic Adventure 2 running in win64 prefix, and this seems to continue with Proton.
or wine should just fix that.
Duplicate of https://github.com/ValveSoftware/Proton/issues/8
WINEARCHx1 2018-08WINEARCH=win32,x1 2018-08
if the user has set e. g. WINEARCH=win32, proton_run will fail to start («wine: WINEARCH set to win32 but '[...]/.local/share/Steam/steamapps/compatdata/221380/pfx' is a 64-bit installation.»
just running «unset WINEARCH» before launching steam seems to work, but proton should probably either support win32 or unset WINEARCH before attempting to launch.