Please read Page 6 of the application firm. " I agree to ........"
The question is not if it is legal or illegal rather you should ask this question:
Would I be in trouble if the find out that I moved and didn't update my address with CIC?
Chances are they may never find out but what if for any other reason you receive RQ and you need to state all addresses you lived at with proves ? what's the proof that you still live in the old address?