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
It’s more like a built-in hardware emulation mode than anything else. Modern ARM chips use out of order execution as the default, whereas x86 uses ordered execution as the default. M-series and Snapdragon X chips have a little flag that can be passed to tell the hardware to run in in-order mode instead of out-of-order mode.