I have lots of experience with this at YYZ at both T1 and T3. Totally different. Not sure how YUL is setup. At T1 YYZ they allow people to enter the transborder area who have come off international flights and not formally entered Canada (i.e. Europe>YYZ>USA). As such to leave the transborder area at T1 for any reason you must (re)clear Canada Customs. That’s why a cancelled T1 flight late in the day is awful.

At T3 YYZ you can simply ask the gate agent to have someone meet you at the top of the transborder area (by the food court) and they will simply escort you out to the landside area through an exit door if you decide not to take the flight.