From the FAQ [0]: - What OS is Steam Deck running? SteamOS 3.0, a new version of SteamOS based on Arch Linux. - Will people be able to install Windows, or other 3rd party content? Yes. Steam Deck is a PC, and players will be able to install whatever they like, including other OSes. [0]: https://partner.steamgames.com/doc/steamdeck/faq
That is a great link. Perhaps even more interesting is this: Valve is working with anti-cheat vendors for Linux (Proton) support. So one of the biggest remaining obstacles to Linux gaming may soon be addressed as a side effect of this.
Though the FAQ says the following, so it might not be quite as bad as I feared:
> We recommend using user-space anti-cheat components for best results, as they can typically run in the Wine environment and provide the same level of functionality. Kernel-space solutions are not currently supported and are not recommended.