Quoting: Eike

Quoting: StellaI thought I read something somewhere about the 64-bit client being unable to launch 32-bit games?

PCGamingwiki says this about the Linux client:

User requests for a native 64-bit client have been denied by Valve on numerous occasions since 2014, purportedly since such a client would be unable to launch 32-bit games.

I wouldn't think so. Your 64 bit environment is able to start 32 bit Steam, why would the chain break if one more application is 64 bit?

64 Bit wine can't launch 32 bit applications.
I think they calculate proton to be part of the Steam client(with which I agree), while the 32 bit trickery of the windows api did come build in with Windows(Win 11 is trying to change this, but thanks to legacy applications like Steam this is currently being maintained)