Originally Posted by
RubeRad
well a modern "11s" derailleur is probably not able to pass an old-school 5-speed chain, plus the last few (5-10?) years have seen a proliferation of cable pull ratios that have decreased compatibility, but yes, most "8-speed" and many "10-speed" derailleurs will work in a Nx9 drivetrain
Yes, 10-speed road will work on a 9-speed MTB, but I doubt it will suit the purpose.
Shimano 10- and 11-speed MTB derailleurs use different pull rates. I suggest a new 9sp Deore (Shadow or non-Shadow), or an Alivio unit if you're trying to keep the cost down.
MTB derailleurs get used hard. New Deore and Alivio are not expensive and it beats buying someone else's problem.