if the player is flying and has flying stamina delta in the negatives

the player should fall and not be able to fly until stamina is regained

Implemented in 21.x.

In upcoming 1.21 versions I'll separate elytra flying and creative-like flying, plus elytra flying with fireworks for more granularity.

Turns out the firework propulsion thing is not bound to player, rather the firework entity is "pulling" the player with it. There's no easy way to track that without incurring overhead, so firework elytra flying state is getting booted :P

Interesting suggestion, flying in the mod is currently being considered as "the creative mode thing you do", with elytra flying also inserted on the back since I didn't think there was a need to distinguish two (I think the movement logic was there long before the introduction of configurable stamina deltas, let alone custom movement states)

In upcoming 1.21 versions I'll separate elytra flying and creative-like flying, plus elytra flying with fireworks for more granularity. I probably won't make creative-like flying get cancelled on creative mode, though. Maybe I'll divide flying state into creative mode and noncreative so users wanting to nerf flying don't have to touch creative mode behavior.