Originally Posted by
cyccommute
For the derailer, the number of speeds doesn’t matter for 7, 8, or 9 speed for the derailer. Once you get to 10 speed, Shimano changed the pull ratio so that it won’t work with the lower number of speeds.
10-speed is the dividing line. For
road rear derailleurs 10-speed is backward compatible with 7,8 and 9-speed components. For MTB derailleurs 10-speed is not backward compatible. 11-speed and above aren't backward compatible at all.