Tried searching but this is not so straightforward so sorry if it's been up before.
The removal of songs from playlists i annoying, but i guess inevitable (withdrawel or not renewel for record company?).
However, i've there was a connection with artist and title in playlist those tracks should be able to reappear since alot of tracks actually makes it back on other albums/releases.
I don't know much about how spotify organize this structurally, but i would be very happy if disabled songs were enabled again or i was notified that the track is now realeased on another album.
One example is Aether - Makishift sanctuary. It was removed some time ago (The album - Artifacts). I discovered randomly that it's back on a different compilation release (Album - Silent In Truth) by varius artirts. Would mean a great deal to alot of playlistmakers i think if it could be an automatic enabeling or some notification at least.