A new beta release for the Steam client includes scoped-achievements.
Achievements can now be split into game mode specific, DLC specific and seasonal specific achievements.
In theory, this will now allow players to achieve achievement completion, even though they’re not interested in the multiplayer aspect of a game.
There’s nothing to “turn off”. It’s just a different way to group/categorize achievements. If you’re already ignoring them then you won’t see anything different.