If you fly the drone 100m up in the air it will block commercial flying due to risk of collision but birdshot can’t reach it.
Even if you manage to hit it at that range there just isn’t enough kinetic power left to really do any damage.
For example here is a Finnish journalist being shot at 70m with birdshot. https://youtu.be/WJgzzrcSmNM?t=124 note that the shot did hit them but none managed to go trough the cardboard and normal civilian clothes were enough protection for other parts of the body.
Basically outside of drones that are trying to hit you (suicide fpv drones) birdshot is kinda useless as there isn't really any reason to fly them so close that they would be in effective range.
Lest anyone misinterpret your statement, shotguns are still dangerous at long range:
No. 7 1/2 shot carries, and is dangerous to humans, for 125
yards; No. 6 shot is dangerous for 250 yards; 3 and 4 shot are dangerous
for 300 yards and BB shot is dangerous for 450 yards.
The heavy shot used for geese is dangerous for 1,400 yards --
almost a mile.
The spread will mean you likely won't hit what you are aiming at, but it is still dangerous.
