SteamOS is Valve’s Linux-based operating system. It features a seamless user experience optimized for gaming, while retaining access to the power and flexibility of a PC, and plays tens of thousands of games on Steam. SteamOS officially ships on Steam Deck, and will soon ship with certain Legion Go S models. We’re currently working on adding support for more devices.
Exactly. Steam OS 1-2 were Debian based. SteamOS 3 is Arch based. That’s a massive difference, and AFAIK there’s no upgrade path to SteamOS 3.