from my own experimening on my 1×10, your 'regular' 9 speed, or road rear mech would work but only with the 10-speed flatbar shifters.
Once you have (dyna-sys) XT 10 speed shifters then you do need a new 10spd rear mech too, not even a 10 spd road mech will work.
Am still experimening though as one of plus points to me of 1x is to have a nice short cage rear mech. The new DynaSys XT rear mech has a seemingly very long cage….
For info, I am using a Sram 10spd road chain and an E13 Guide Ring