On their WatchOS page, about halfway down: https://www.apple.com/os/watchos/

Has to be a fuckup. watchOS 26 supported the Series 6 onwards. I don't think Apple has ever cut 3 generations of devices with a single yearly OS upgrade unless there's an underlying architecture reason for it before.

This might be the list for Siri AI supported watches or something.

Seems you are correct, the beta is now showing available on my Series 9 at least.

wow this blows. do they ever like release an OS for older machines at a later date?