While I could see allowing us to mount and ride on the ground just like Idyllshire and Rhalgr's Reach, I'm fairly certain flying won't happen in S9.

We tend to forget how much of what we see in a game is more 2D illusion than 3D models. If we could fly right now, we'd probably be seeing a bunch of empty holes from above. Ground mounts would be as far as they would go since our perspective wouldn't change sufficiently.

As for just disabling mounting in interior areas, technically the entire zone is interior. But yes, if mounting was allowed then those smaller structures we enter would desperately need a "Hoofing It" debuff like we get for the Moonfire Faire tower.