Songs imported from cloud services and DRM-free songs downloaded to the Music app can have their key changed. However, Apple Music songs, DRM’d songs and songs from your iCloud Music Library that have not been downloaded cannot be transposed.
Because not all songs can be transposed, these controls are now hidden by default to provide a consistent set of features for all music. To re-enable these controls for compatible songs, use the Settings app, locate the Perfect Tempo section and switch on ‘Show Transposition Controls’.
When enabled, these controls can be accessed by swiping left on the album artwork on the playback screen in Perfect Tempo.