Yeah, mostly. Some of the projects they use are licensed under non-copyleft licenses (e.g. dxvk is under zlib which isn’t copyleft). Valve pushes many parts that constitute SteamOS into their own forks that then trickle into the mainline projects, e.g. Proton is opensource and changes to Proton’s version of Wine are slowly introduced into mainline Wine. You could slap all these changes together and compile your own SteamOS, but Valve currently doesn’t publish it as one nice package. There are projects like Bazzite that apply these changes to other distros (in this case Fedora).

The old SteamOS from 2015 was indeed publicly available. It was to be used on the so-called »Steam Machines« and it was based on Debian.

The new SteamOS from 2022 is based on Arch, is made specifically for the Steam Deck and is not available publicly. Some similar distros exist.