Some of the newer mobility scooters have regen braking. Some have regen and electromagnetic braking. But I'm not sure if regen works downhill.
I've been studying a few youtube videos on scooters, but all I have been able to find out, is how the brake works. It's a solenoid brake, on the end of the motor shaft, requiring to be powered to release it. From full speed, snap releasing the lever, it seems to slow down quite gently, then the once at a stop, the click, as the brake comes on - if on a slope, it then rocks back an inch or two. Which suggests either regen braking, or a worm-drive - using the motor speed to control slow down. So the brake disk, is really just the equivalent of a handbrake.