Actually it's possible to add spectate for drone but it's not worth it.
Explanation:
We can change player's camera position but it wont be as smooth as the built-in spectate system, to improve it we have to change player's camera very frequently, do more calculation( to make sure cam is behind the moving drone ), add ability to control camera..etc, Besides that, the drone currently also pass through building/walls which is unlrealistic to spectate( you will see inside building..etc ) unless we do another additonal calculation to check rotation, movement, obstruction..etc and control the drone movement not to hit obstruction or pass through it. But i think it's not worth to add it for such a minor feature and cause alot more work for server to handle...