SteamOS was available to download for free, like many other GNU/Linux distributions out there, and you were able to install it on any machine if you wanted to turn it into a full gaming computer. I personally followed its development, and the latest version ever released was SteamOS 2.195, more than two years ago.
Apparently, during this time, Valve rebased their SteamOS distro on Arch Linux, a powerful and flexible rolling-release distribution, most probably to provide users with the latest security and software updates as soon as they are available upstream.