Blizzard confirmed they’re working on implementing full Dragonriding in old zones, but it won’t go live in Patch 10.1.5.

(Source)

Good day dragonriders,

As data-miners have guessed, we are indeed working on dragonriding outside of the Dragon Isles and broadening the set of mounts that can dragonride. Our initial work on this can be seen in the Fractures in Time game files, but that feature will not be complete or featured in the live game until later in Dragonflight.

As we continue to develop the game in this way, we expect there to be many examples in the future of content that is a work-in-progress becoming visible to data-miners in test environments. We intend to announce the contents of any given patch in our official posts about that patch.

For the latest details on what’s included in Fracture in Time, please check out our official development notes.