@Jain @hackernews > Steam Deck runs SteamOS version 3, based on the Arch Linux operating system. While SteamOS had been previously developed for Steam Machines using Debian Linux, Valve stated that they wanted to use a rolling upgrade approach for the Deck’s system software, a function Debian was not designed for but was a feature of Arch Linux.[29] An application programming interface (API) specific for the Steam Deck is available to game developers, allowing a game to specify certain settings if it is being run on a Steam Deck compared to a normal computer.[30] Within the Steam storefront, developers can populate a special file depot for their game with lower-resolution textures and other reduced elements to allow their game to perform better on the Steam Deck; Steam automatically detects and downloads the appropriate files for the system (whether on a computer or Steam Deck) when the user installs the game.[12] – https://en.wikipedia.org/wiki/Steam_Deck#Software, see license and attribution at url
It does run Arch, unfortunately actually.