> Steam game updates are mandatory and you can't downgrade the game to a previous version either.

For Crusader Kings III, the old versions are listed as betas (cog -> properties -> betas) so you downgrade by "signing up to a beta".

I don't know if it's a common practice but pretty damn necessary for paradox games. A single game might take months and their attitude to backwards compatibility is "new versions will corrupt your game files in ways that only subtly reveal themselves like noticing the King of England owns a county in Mongolia before reaching a game year that will always crash".