This is categorically wrong. I have been able to successfully do this for multiple games, the most recent being KSP when an update broke all my add-ons so I downgraded until they all caught up.

Looking at KSP, this is a courtesy of the developers since they decided to keep other versions available as _beta branches_.

This is certainly not the common case. Look at other games, Doom Eternal, Hollow Knight, Arma 3, Divinity: Original Sins 2, ... They don't offer that.

Look at the automatic update settings. There is no option to not update a game at all such that you can keep playing the current version. At any setting it will still check for updates when you launch the game (for the first them since starting the Steam client).