[2023] Freeze a mod playset

I don't believe so; Steam automatically updates all content - games and mods - at the latest when you next go to play any game. You'd basically have to put Steam into offline mode, or disconnect the computer from the internet.

dehen said: I know how to freeze the game version in steam

Actually, you can't do that either! All you can do is choose between the update downloading soon, or when you next play.  

dehen said: Well, can't you freeze to an old version under the 'betas' section in steam?

Oh, an old game version, yes. But you can't freeze on the current game version and prevent Steam from applying some future update to it.