That is relatively new, though. As far as I remember, in the "old days" as a foreigner you got those cardboard slips stapled into your passport which were removed again when you left the country. Today they just do this electronically and keep track of who is in the country by receiving your passport information from the airlines. (You can even check your arrival and departure history online: https://i94.cbp.dhs.gov/I94/#/history-search)

That link didn't work for me. It said no records found, but I’ve left the USA several times in the past 12 months. Weird.

The US stopped doing it around 2014. They also stopped doing it in JFK before they stopped doing it in LAX; so when I flew in one side of the country and out the other I had to argue with the clerk....

Edit: I see wikipedia mentions the move to electronic record keeping instead of adding it to your passport: https://en.wikipedia.org/wiki/Form_I-94#Automation_at_sea_an...

Are you a U.S. permanent resident or U.S. citizen?

It only works for foreigners, I believe, as U.S. residents don't need I-94 forms (the old cardboard slips). I'm also not sure if it works for Canadians, as Canadians can also travel to the U.S. more easily.

Even though CBP doesn't check any papers, airline agents swipe your passport or record your passport details. And these details are sent out to CBP.

Canada doesn't either. We just have big signs warning people not to try to bring pot with them because it's probably illegal wherever you're going.

The only ways to leave Canada are by air or to the US, and in either case your departure will be shared with the Canadian government by either the airline or the US government. Among other things, it prevents residency fraud (claiming benefits like healthcare or old age pension while actually living abroad).

Countries that lack strong bilateral cooperation with their only land neighbour have to use other methods, like exit controls.

No ships?

Mexico didn't check me on the way out. Then again, it probably depends on the port of egress.