Supposedly they were working on an inhouse one, and then changed directions to just do software tweaks for other companies hardware instead.
It definitely needs some background process limiting, comparing official windows vs official steamOS on that Lenovo legion Go showed pretty terrible performance impact from running windows.
I'd recommend looking at the articles I linked, I probably should have refreshed my memory on them better before commenting.
In addition to knowing that valve is working on compatibility layers for running x86 on arm devices, there was also a steamVR update 9 months ago contains files for an ARM device code named deckard. There's probably more relevant leaks too, I think some renders of deckard controllers got leaked at some point as well.