Thank you. That's interesting and actually supports the assertion that some executables are still built in 32bit, this reaper thing in particular.

It also lead me to a requirements list explicitly containing the following:

Architectures:

• x86_64 systems must support multilib and have both 32-bit (x86, i386, lib32) and 64-bit (x86_64, amd64) versions of required libraries and graphics drivers installed
• 64-bit-only x86_64 systems are not supported
• 32-bit-only x86 systems are not supported

It's here: https://github.com/ValveSoftware/steam-for-linux/blob/master/README.beta.md

Found via this comment from what seems like a valve employee: https://github.com/ValveSoftware/steam-for-linux/issues/13015#issuecomment-6204720307