Can a 11-speed derailleur be used with a 9-speed shifter? Can a 11-speed derailleur be used with a 9-speed shifter
I want to buy a 11-speed derailleur and have  a 9-speed shifter so will they work together or not?
With Shimano, no. The cable pulls are different. You might be able to futz with a JTek Shiftmate to get something working, but the cost would be way higher than just buying a 7-9 speed rear derailleur.