SteamOS

SteamOS is Valve's Linux-based operating system. By default, the Steam client serves as a user interface and provides connectivity to Steam, but the user can switch into the Desktop Mode and access an Arch-based desktop with KDE as the desktop environment.

Valve partnered with Collabora, a private open-source software consulting company, to make the software. Version 3.0, which Collabora helped in modernizing from Debian to Arch, implemented the immutable file system, with only the user's home directory being writable.

Links
SteamOS Wikipedia page