Does Shimano XTR RD-M9000 SGS 11-Speed Shadow+ MTB Rear Derailleur Long Cage work with 10speed XT shifter?
I can say from experience this does not work well at all.

Although as mentioned above the chain does run through the shifting is dreadful. I tried it with a 10 speed shifter but gears would always have a spot where they would jump.  

Just get a 10 speed rear derailleur.  You will be glad you did.