Apple - Is handoff / continuity (iOS 8+) supported in iTunes to seamlessly switch playback between Mac and iOS?

Handoff is not supported in Music.app, however your use case (control media playback on your iPhone from a Mac) would be more along the lines of what the Apple Remote app does, just in reverse.

Tunes Remote allows you to control one device from another device or a Mac. I have not used the software, so YMMV.

As several other people have already answered, no, at this time iTunes and Music do not support Handoff.

However, there used to be a piece of software that did just this, called Seamless. It was removed from the App Stores, though, after “Apple breakingly removed a component from iOS that Seamless used to allow for iPhone<->Mac communication.” The developers say they “can work around it, but it'll take some time.” Maybe it will come back at some point, in which case it could be a good workaround.