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
In the shorter-term the issue is the lack of sufficiently powerful commercially-available RISC-V hardware for the level of gaming people expect out of a Steam Deck or VR headset, which ARM already has a number of SOCs capable of.
I don’t doubt that the work will continue but Valve isn’t likely to pour time or money into it until they think the hardware is there.