Up to Pro Tools 12.6, I could delete empty playlists by simply switching from the playlist view to the waveform view and then back again.
The playlists without audio content would be gone. This feature was extremely helpful for things like huge loop recording efforts - say you'd have five vocal phrases that would be looped onto ten playlists each. That's fifty playlists which could then be combined onto five playlists. With the feature I'd described, I could then just go to waveform view and the unused 45 playlists would disappear. A great feature that should be reinstated (or altered to be an option).