From what I’ve read, syncing via iTunes to iOS devices will continue ( look at the 2019 WWDC info. to satisfy yourself).
Good news that Audirvana 3.5 is compatible with Catalina and the Music app.for those interested.
Not sure whether internet radio will be available in Catalina.
Personally, happy to remain with El Capitan for music as it has direct mode available with Audirvana 3.5 and continue to access internet radio.
Sidecar is kinda interesting for running a Mac music server headless with access via aniPad (not essential as there plenty of great screen sharing apps out there)