

It’s been a thing, not specifically just Canada
https://www.wired.com/story/phone-searches-at-the-us-border-hit-a-record-high/
That said I don’t personally know any canadian citizens that have traveled in/out of the U.S recently. Not too sure if it relates to OP’s question, OP never mentioned canadian citizens only that they has a GF that goes to school in Canada.
Not sure about the error but I kind of doubt modernize-sources would make/break your upgrade. I did a clean full install of Debian 13 and the sources file wasn’t even modernized on that, it’s still the old version. Would be very surprised if somehow sources is following a different requirement for a Debian 13 upgrade.
From the error maybe your version of apt doesn’t support the “modernize-sources” option… but that itself isn’t relevant to your Debian upgrade.