I used to fly model aircraft, I have some ESCs way back from 2011 or so which did this. So it's been around for a long time.
If I recall correctly it's implemented by just driving 2 out of the 3 phases so the motor doesn't commutate. You don't normally notice the motor whine when a prop is attached on account of the prop making a lot more noise.
Even the prosumer DJI drones do the same thing with their motors, they don't have beepers built in, signals all sorts of things like ready to fly.