& Shimano's 10 lacks the friction option of the 8&9.. They make them for tri and time trial bikes , touring builds just adapt.,

there are advantages to not catching upgrade-itis.. as said above, some build specs use the 9 MTB RD
as its Usable With the bar end 10 & cassette.

but whole 9 speed drive train should be fine (happily toured with a 3 by 7, for decades , myself )