I'm sure you all know the drill, because this has been complained about for well over a year now.
Offline playlists sync to the iOS device, and all is well in the world. No issues here. The issue comes when you decide that you no longer want that playlist available offline. You set 'Available Offline' to 'Off' and...the files remain on your device. Forever. Or at least until you run out of space and have to delete then reinstall the app, then resync all the stuff you actually do want.
Guys, this is not a solution to a problem this basic. My original thinking was that it left the files on there to avoid resyncing if you changed your mind, but then allowed them to be deleted when iOS goes into 'Cleaning...' mode on apps to delete cache and free up space. I don't know if this is the intended behaviour, but it does not happen. App downloads are failing because Spotify is sucking up gigabytes of storage, when the playlists I actually want offline amount to only a couple of hundred megabytes.
Android has an option to delete cache. Deleting, reinstalling, and resyncing again is not a fix for this problem. It has been reported by users for at least a year.
Please fix it.