I would expect the devs to have taken inspiration from WoW with regards to flying mounts. If you flew too far out of bounds of the map you would get a stamina bar counting down until you returned within the border of the region. If the bar fully depleted your mount would be exhausted and auto-dismount you, causing you to die. It's a much less intrusive way of keeping the player within the boundaries of the map and would actually be much easier to implement than invisible walls everywhere in 3D space. Hopefully we'll have a similar system here.

One thing I am curious about though is what would happen if you dismounted over a deep pool of water like in the Dravanian Hinterlands... since we have no swimming yet I assume you just automatically die but I'm curious to see how they handle it nonetheless.