Can't add songs to collaborative playlists on Spotify Web Player since a recent update - unless I'm missing it somehow. Either way not obvious at all. Can still do it on phone. Seems an incredibly basic thing to overlook in an update.
Thanks for reaching out to us on the Spotify Community. We hope you're well 🙂
There's recently been an update, and a few changes have been made to the web player. The team here are still working hard on things, so we appreciate your feedback on this. We'll pass this directly on.