Was a gradual shift after 9/11. US started requiring passports (on paper, but if you're a US citizen entering without one, I don't think they can do much more than give you a hard time). Lots more quizzing upon entry into US.
Good ole days (pre-2000) meant giving a verbal declaration of citizenship and being sure to have a decades old crumbly birth certificate if they asked.
Canada technically doesn't require passports at the land border, but they make it more of a hassle if you don't have one.
On the plus side, the Canadian side cares a lot less about whether returning Canadians owed taxes & duties. The booths used to be staffed by a division of the CRA (Canadian IRS) and unarmed.