In the common ESCs used in drones, they don't care about rpm. The IMU measures rotation and acceleration in x, y and z axes. These signals are used in the PID loops that control tilt, pitch, yaw and vertical movement. The controller varies PWM on the ESCs to control motor power, and the resulting rpm is whatever is needed to produce the thrust required to stabilize the drone.
For full autonomous control the drone also needs absolute measurement of altitude and heading, which is provided by an altimeter (air pressure sensor) and magnetometer. It may also have a GPS for positioning control and to follow waypoints, and infrared or ultrasonic distance sensors for collision avoidance.
Note that while there is a relatively linear relationship between PWM and thrust, it is also affected by air currents. When the drone is hovering the propellers create circular air movement which reduces thrust. This causes the drone to sink when hovering in one spot, and rise when moving into 'clean' air. To maintain lateral balance the thrust of each propeller has to be precisely matched, which requires constant ESC throttle adjustment. Without feedback the slightest amount of thrust imbalance would cause the drone to flip over instantly.
One application that may set a 'target rotational velocity' is the main rotor of a collective pitch model helicopter. Having closed loop control of motor speed stops it from 'bogging down' under heavy load, making the collective control more responsive and precise. An ESC designed for this use may have a specific target rpm programmed into it, or it may allow setting open-loop motor speed with the throttle, then locks into the resulting rpm until the throttle is changed.