“Applications with existing extended mode Web API access that were relying on these endpoints remain unaffected by this change.”
Spotify cut Related Artists, Recommendations, Audio Features, Audio Analysis and 30-second previews for new apps, effective the day it was announced. Apps still in development mode lose them too. The stated reason is a “more secure platform,” with no word on how hiding Related Artists secures anything. Whoever already has access keeps it, and the next developer gets the door.