I'm pretty sure SteamOS is just an outdated Debian distro.
It is a debian derived set of software in part, but it has a bunch of other drivers and such that are entirely Valve's stuff. It is not outdated, they make updates to it plenty. The priority of it all is to run on recent hardware and to play games, it is what they have on their steam deck and steam machines.
SteamOS2 was dropped, SteamOS3, which is used for Steam Deck, is based on manjaro linux, which is an arch variant.
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