Another consideration is what happens on the other side of the journey. Canada, Britain, Australia, and New Zealand, which all share intelligence with the U.S. government under the Five Eyes program, have different border privacy policies. Under Australian law, travelers do not have to unlock their phones. Canadian authorities, like U.S. authorities, say they will seize a phone if a traveler refuses to unlock it. New Zealand imposes a $5,000 fine for failing to unlock a phone, and Britain considers refusing to unlock a phone for police to be a counterterrorism offense.

Quite insane. Seems like the only real answer is a burner phone.

Uh wow. Last time I traveled outside of Europe, it was still pretty safe. Seems one good option would be to make a cloud backup, wipe the phone before the flight and restore everything using the first hotel wifi. I wonder if creating a second (empty) user account on the phone and unlocking that one at the border is enough to comply with the law.