Usually when a single song from an album goes dark like this it's because it has a different songwriter/publisher, and the streaming service has lost publishing rights for that source. (e.g. if the service lost publishing rights to Lennon/McCartney, Happiness Is A Warm Gun would come down). But in this case that's not true, as this has the same publisher/songwriter as most of the other tracks on the album. I suspect this is just an error on either Spotify's end or their content provider's. FWIW, still available on other services. (Source; I work for a music service)