As far as I know with active braking option enabled in BLHeli32 the speed controller will counteract the free spinning of the rotor according to throttle value hence making it slow down quicker when needed. Could this help protect propellers upon crash?
No matter how quickly you slow down the propellers after an impact with the ground, much of the damage actually comes from the impact directly into the ground and the force from the mass of the quad, so the speed of the props shouldn’t make too much of a difference.
If you mean the slowing down the props after it’s on the ground would save them, it may prevent further damage to the props, however the current draw if the props get caught in something could easily fry an ESC so it is likely more economical to sacrifice the props.
I generally go by the rule that if I crash, disarm as fast as possible. This stops me from burning ESCs as I’m usually disarmed before I hit the ground, though this comes with the negative that i can’t recover after a crash because I’ve disarmed.