Old_stable also has Crossplay, so it won't help you anyways (it only gives you the most recent previous version rather than holding to the last known stable version).

This reddit post provides the info for downloading older versions of Steam products:
https://www.reddit.com/r/Steam/comments/611h5e/guide_how_to_download_older_versions_of_a_game_on

When searching SteamDB, the title to looks for is (obviously) "Valheim dedicated server". From Depots, you want "Valheim dedicated server Windows" or "Valheim dedicated server Linux" depending on which OS you run the server on. The Manifest you want to look for is the one from September 30 - This is the last version from before the addition of Crossplay (0.210.6).