I'm going to guess what "happened" to flying tank mounts is the exact same thing that happened to:

- Roaming, capturable primals.
- FC alliances.
- New egis (glamour).
- male-equivalent bunny set.
- minions idling in player rooms/houses.

People could probably think of other things that were mentioned somewhere at some point, but I'm betting a lot of things that were "being looked into" or "will be coming" were either quietly dropped, hoping to never be brought up again or are so far down the to-do list they very well might be saved for patch 4.55 hype content.