I was prompted to upload test results for my Saturday YYZ-EWR flight during checkin (using the mobile app). I did so and they were approved - but an Air Canada agent standing outside the customs hall collecting attestation forms still asked me to show it to them at the airport anyways - so not sure what the point of that was, but presumably at another station where AC doesn't staff the door to the customs hall, I could have got through without ever showing the results to anyone in person.
Was not prompted to upload results for my BOS-YYZ return flight yesterday. Showed the PCR test and vaccine proof at the gate (still not sure why they asked for that and not the ArriveCAN receipt).