"SteamOS is our Linux-based operating system. The base system draws from Debian 8, code named Debian Jessie."
Debian Jessie is terribly out of date and hasn't seen regular security updates for five years now, from https://www.debian.org/releases/jessie/:
> Debian “jessie” Release Information
> Debian 8.11 was released June 23rd, 2018. Debian 8.0 was initially released on April 26th, 2015. The release included many major changes, described in our press release and the Release Notes.
> Debian 8 has been superseded by Debian 9 (stretch). Regular security support updates have been discontinued as of June 17th, 2018.
edit: it looks like the SteamOS build for custom hardware is no longer available for download, as the download link now leads to a "Download SteamOS: Steam Deck Image" page.