>It's not up to Valve.
Why not? They develop it. They can also release it.
Isn’t it available on https://store.steampowered.com/steamos ?
That's the old Debian-based Steam OS 2. The Steam Deck uses Steam OS 3.0 (Arch-based) which is not yet officially released.
In the meantime, there's HoloISO [0], but it's unofficial.