This would presumably let x86 windows games run on ARM hardware.
This is almost certainly meant for the next Valve VR headset, but ARM has so much better power efficiency than x86 that a future ARM based Deck would be a huge improvement to battery life.
Also see this tweet:
VR games that have already secretly pushed Android ARM builds onto the Steam Store are ran via Waydroid (androidARM to LinuxARM)
VR games that do not have an ARM build on Steam (windows x86) are being translated/emulated via ProtonARM and FEX
Edit: here’s gamingonlinux coverage of this info, includes some more information
Part of how they’re identifying that proton arm and steam Waydroid exists is that the tools are being used to test VR games uploaded to steam, or were uploaded in a batch of other VR assets.
I fully hope to see this apply to Steam Deck/Chromebooks/Android/etc, but right now any hints of these have been VR specific. We haven’t seen the Proton ARM before, but previous leaks about Waydroid have also all been VR related.