Are you asking the other-way-round question, i.e. can you use a 10sp rear mech with 9sp shifters/cassette/chain ?
Yes.
The front and rear mechs don't 'know' how many gears you have.
The shifters pull a certain amount of cable, the mech moves that much sideways.
A 10sp shifter pulls slightly less cable than a 9sp shifter, which in turn pulls slightly less than a 8sp shifter.
The mech moves as far as it needs to, the amount for a 10sp or 9sp or 8sp cassette respectively, because the shifter has pulled the right amount of cable.