Valve’s failure to release a version of SteamOS 3 to the general public looks like a major oversight, especially as an earlier version - based on Debian 8, aka Jessie - is still hanging around with a warning it’s not compatible with Steam Deck. The available version is a hangover from Valve’s earlier flirtation with hardware, the Steam Machine, and even comes with instructions on how to build a DIY machine. Otherwise, all that’s available from Valve is a Steam Deck recovery image.
The version our Steam Deck-owning friends are using is no longer based on Debian (a version based on Debian 9 was canned last year), but instead uses Arch Linux as its foundations. It contains the Proton Windows compatibility layer, and goes by the codename Holo.
Hence the name of HoloISO, which can be found on GitHub as a repository from Adam Jafarov. “The code, and packages are straight from Valve with zero possible edits. [The] ISO is being built on [the] official Steam Deck recovery image running inside [a] QEMU instance,” the Readme file reads. There are a few caveats for anyone wanting to try it out: the ISO only boots if written to a flash drive with one of four apps (BalenaEtcher, RosaImageWriter, Fedora Media Writer or dd with 4MB block size). It doesn’t play well with Nvidia GPUs, requiring proprietary drivers, and Intel Arc or integrated GPUs aren’t looking great either, requiring a tweak to Gamescope and MESA.
What does work, assuming you have the compatible hardware, include the first-boot experience, Plasma desktop (including Valve’s Vapor skin), the Deck interface, and behind the scenes stuff like shader precaching, neofetch, and FPS limiting.
It might be worth waiting a little while to see how quickly the project solves its current problems, however, as the latest release, known as Hallway Pizza, is “probably” the final beta before a stable version is released.