Hey Spotify forums, I just noticed that one of my favorite Borgore tracks got changed. Miley Cyrus doesn't sing in the opening chorus anymore and it's really irritating me...what happened to the original? 😕
Any content that is missing is normally down to the record label or rights holders with holding those tracks from Spotify, sometimes to push up physical and download sales. There are also some big names, such as AC/DC who have chosen not to be part of Spotify.
Any content that has disappeared or been removed (the tracks will still be in your playlist, just hidden because they unplayable) is normally down to the record label or rights holder. Sometimes content is removed due to licensing changes, transfer of ownership or due to a request from the content owner. You will find if content gets removed Spotify will automatically try and link it to the closest match in the catalogue which can often be the same song on a different album.
Have a read at the Music FAQ topic for a little more information on how music is added to Spotify and a request form.