How do I reset Apple Music library without affecting old iTunes library?

You cannot really reset your Apple Music subscription unless you cancel it and wait the 30 days or so that it takes for Apple to do it automatically. What you can potentially do is hold down option when launching Music to create a separate library that will initially have no content except your unhidden purchase history shown in the cloud, you can then enable sync library to see your active cloud library content. Delete all of the items and playlists in that that you don't want, then reconnect to original library. Any local content that was removed from the cloud library will be marked as removed, but can be added back if you want to keep it. Cleaning in a separate library, or if not that then backing up your library first, is advised as it is all too easy to be over zealous when cleaning up.

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