Steam should really just let you set games not to update without having to futz around with Offline Mode. Refusing updates for specific games is currently possible-but-very-annoying: you need to set “Only update this game when I launch it” and then ALWAYS switch Steam into Offline Mode before launching; if you ever accidentally launch it while online, it will update and Steam won’t let you roll back.
Only update this game when I launch it
Yeah there should be more steamwide options for this
properly allowing selection of game versions is a work in progress tho :) https://www.gamingonlinux.com/2024/11/steam-gets-new-tools-for-game-devs-to-offer-players-version-switching-in-game/