Servo Function 22

I have an agro drone running on Pixhawk Cube FC with Arducoptor 3.6 running. What I have noticed that the servo function 22 doesn’t exist. When You Choose the function 22 the pump doesn’t work. but when u select the RC pass through the pump works all fine but at a constant speed. What servo function 22 should be doing is to adjust the pump speed as for the speed of the drone. does anyone else fixed with the issue of the servo function 22? its a very useful function and have no idea why its has taken out