Customs has always had discretion for who it will allow into the country at ports of entry.
I'm an older GenX Canadian. This is of course correct.
The difference now compared to 20 years ago is the outright hostility you encounter more often than not from American Customs and Border Protection agents.
While never particularly friendly, in the before-times they were very rarely antagonistic. It was an efficient, almost boring exercise.
Contrast that with now while they angrily scroll through your phone in case you have JD Vance memes while barking orders (yes, they could always look through your phone, but they were looking for CSAM or evidence of immigration violations, not whether you had an AI-generated image of Putin and Trump smooching.)
Canadians like me are saying "screw that" and going to Costa Rica, Mexico, Europe or a hundred other places instead.
We're off to the Netherlands on Thursday. Two years ago we would have gone to the Eastern Seaboard.
Of course the Trump supporters reply "Fuck you, America doesn't need you anyway" but the fact remains that across the USA there are campaigns desperately trying to get Canadians to come back. To which we respond "Fuck you, America."