I've just tested several "unknown" areas, and it seems you can no longer cast flying mounts in Unknown areas.
Regular mounts can still be cast and used. Just hybrid and flying mounts disappear the moment they are cast. I attempted a DC during the cast, logged back in still on the flying mount, but dismounted the moment I tried to move.
This means no more flying behind Zul'Aman, outside of Deepholm, the edges of the Draenei starting zone, and other such areas.![]()