I am 99% sure this has already been suggested but it is such a key aspect to Spotify am desperate to have it back! I need Spotify to take notice of it so I ask you to kudo it.
The activity feed sidebar essentially makes Spotify, Spotify. The reason I don't use the other platforms is because they aren't "connected". I can't discover music in the same way. I want to be able to find out what my friends are listening to. I'm sure it also helps artist get more plays.
The way I would do design Spotify is fill it with as many features as possible and then have a tab where the user can pick which features to enable. Why would you remove features? Can't you just disable them?
So, please kudo this, hopefully Spotifty will take note and reinstate this feature. A lot of the community is asking for the feature!