Back in August 2012, Valve rolled out some community updates, including the "Friend Activity" page, which amongst other things can list which achievements your friends have acquired, which games they've purchased, and community content they've submitted:
Is there a method to hide my activity from appearing on my friends' "Friend Activity" pages?
64 Answers
It's not an ideal solution, but you can hide your public activity by setting your profile to private. Note that your previously posted activity may remain in your friends' feeds. (Thanks to OP for testing this.)
Go to activity then by the top right of your screen. You'll see '' Friend Activity Settings ''
You cannot hide your Recent Activity counter (Hours past 2 weeks) I haven't found it.
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.
you can hide your activity in "friends activity" "friends activity setting"
1
