You are not in control. Neither can't you downgrade a game, nor can't you disable the update mechanism.
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).