Music is not coming back to the Plex app, you need to use PlexAmp for music. I did a search on the forum and deleting is a common request going back 4 years or so…so I doubt it is coming soon.
Music and Photos: We have built dedicated music and photo applications that are companions to the Plex app. Plexamp and Plex Photos are where access to music and photo libraries will live moving forward. As such, music and photos access will no longer be available within the regular Plex application when it gets updated to the new experience. Both companion apps are still in active development and can expect updates soon.
https://www.plex.tv/blog/the-future-of-plex-focused-streamlined-and-ready-for-feedback/
https://forums.plex.tv/search?q=plexamp%20delete
After using Plex for quite a while I have just started using plexamp. It is the best player by far and I love the sonic adventure! The only thing I would ask for is the ability to directly delete a track from the library. Currently if I decide I don’t like a track I have downloaded, I mark it as one star and then I will delete it in Plex
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