I definitely think the ability to sort playlists (not the songs within playlists but the actual "custom order" option) should be made available on mobile, especially since the option to display in custom order is given.
Updated on 2020-02-07
Thanks for sharing your idea here in the Community!
Keep in mind that something very similar has been suggested here: https://community.spotify.com/t5/Live-Ideas/Mobile-Custom-Sort-Playlists-on-App/idi-p/4812306
We'd recommend heading over to that idea and adding your +VOTE and feedback there.
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