IIRC the argument used for this is that since you have to go through customs at an airport for international flights, the airport counts as a “border”.

I’m so glad I never got around to doing one of these tests

Larger denominations will never be issued. Almost all financial transactions are digital and traceable these days, whether it be to investigate crime or more effectively market to you. The powers that be will never give that up. All they have to do is wait for inflation to erode away the practicality of carrying cash, at which point no financial dealings will ever happen without your Visa/MC/Amex masters knowing about it.