I saw this issue raised in the Aeroplan forum (because I'm new, I can't post links, but search the phrase "changing domestic connection flight after int'l arrival?"), but I think it's broader than that, and the answer wasn't entirely clear.

Basically, Air Canada is still selling tickets from international destinations to Canada, with a domestic connection after (for example, SFO-YUL-YYZ). My question is, what is going on with the domestic portion? I imagine that at immigration at your arrival point (YVR/YYC/YYZ/YUL) you'd have to describe your quarantine plan. But, according to the Air Canada site, you'd then proceed to the domestic terminal and fly to your final Canadian destination. (The "airport information" pages on the Air Canada site for arriving from the US, make no mention of quarantine hotels). Presumably your bags would be checked through too.

Would you be "intercepted" while walking to the domestic terminal? What about your luggage? And how could there be any follow up, once you arrived at your final (Canadian) destination?

Either Air Canada has not updated its booking system, after all these weeks of the quarantine policy; or this is a massive flaw in the government's quarantine plan (which is equally, if not more, plausible; and which, for the record, I think is just a political stunt).