Everything's ultimately based on a rolling distro; Debian is frozen from Sid and Fedora (and ultimately RHEL) are frozen from Rawhide.

Agreed. Valve is the maintainer, it's no longer 'rolling' (or Arch) by virtue of creating SteamOS and delivering momentary snapshots. Arch is upstream and largely irrelevant to us, the consumers of SteamOS.

Ask anyone outside of Valve or strange hobbies what packages actually 'make' SteamOS; they couldn't tell you. It's simply not their concern.

In fewer words: if you need/want to know the lifecycle, you'd know. This "by the way, they're a year old" message is in bad faith. The Fedora release that equates to any EL derivative is comparatively ancient. It's fine.