On older DJI Phantoms, you could connect to the Wifi hotspot and SSH into the drone. Not sure if that's still possible.

Yep, and not so much recent example : Tello, bebop... Those are quite old and well open to allow external control from custom softwares

Not many, but that's just PHY stuff. The threat model is really the barycenter of these targeted offensive security platforms. Adding support for new protocols is generally trivial if there's an option, and a place to leverage them will tend to incentivize development when there isn't.

No, adding support for new protocols is far from trivial. There are so many factors that can make this non trivial such as encryption, channel hopping, DSS, hardware required, etc.

I spent about eight years working alongside a software defined radio project. I saw the effect first hand. If you build an ecosystem in which folks with the appropriate skills can make a contribution, they tend to show up.

The main challenge is licensing, not technical siphistication. Most of the protocols in question are quite a bit less complex than wifi or Bluetooth.

“Just PHY stuff” is a pretty big barrier though isn’t it?