Phoronix article: https://www.phoronix.com/news/Steam-Machines-Frame-2026
Also listed here: https://store.steampowered.com/sale/hardware
Valve has already sent support for the new Steam Controller upstream: https://www.phoronix.com/news/New-Steam-Controller-SDL
It days right in the marketing text that the headset is “a PC” which to me implies full SteamOS distro with no limitations on installing a different OS, if you can get the many hardware drivers to work.