Add button removed / no plus button anymore

[AresMsc]

Guess you know, what i mean - > the button, that replaced the heart.

I really loved the functions, it brought. Like adding songs to multiple playlists or looking, where i have added the song already. 

I also think, this is an important feature and i cant relate, why spotify added it this late (soundcloud had it like for ever).

Heres the Problem:

This feature came in late february and got removed a few weeks after. 

Now again, it came back a few days/weeks before and today its removed again.

Why would you do that? Its so frustrating to get back to such a limited function, as the heart has.