I was just digging into some settings within my Steam profile and identified how to do this. Couldn't say when this feature was added, but it does seem to fit the bill for the question.
Navigate to your profile -> Edit Profile -> Privacy Settings -> My Profile -> Game Details
As per the description of that setting (highlighting mine):
This category includes the list of all games on your Steam account, games you’ve wishlisted, your achievements and your playtime. This setting also controls whether you’re seen as "in-game" and the title of the game you are playing.
Setting this to private will also hide your currently active game from your friends, but it will also hide your achievements and playtime from their feeds.