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I would like to add my vote for a playlist folder feature.

I have so many playlists for different projects that a flat, non-hierarchical list is messy and cumbersome.
It's almost as bad as back in the early days of Apple Macs filesystems that also couldn't support folders/directories.

As far as most feature requests go, addition of a playlist folder/expandable/collapsible playlist organiser would see one of the simplest and beneficial.