There is no way to massive delete playlists, but what you can do is make a new playlist folder (File > New Playlist Folder) then drag and drop all of your playlists into it and then delete the folder. It saves you having to do the confirm window each time. I have asked before for others and the Spotify team can't delete playlists for you to my knowledge.
Use soundiz. It helps you keep all your playlists across many platforms synced. Keeping a Playlist synced with YT Music and Spotify for example is a premium feature. But you can just have all your playlists synced to soundiz itself and I was able to delete like 200 to 300 playlists all at once. You just select whatever Playlist you want and delete/unfollow them. Apparently this deleted them on spotify also thank God. Thought it would have only unsynced them.