I have TW3 on Steam and i won't be able to not update the game like on GOG, but you could allow people to select that old version of the game (the current one) instead of the new one.

There are tons of amazing mods to use on TW3 right now and i don't have a 10k PC to run your ultra high end version of the game, i don't wanna be forced to update the game and be stuck playing with LOW settings since right now i can play the game with 60fps + HIGH settings with my current PC.