I recently went back and forth across the Point Roberts border crossing. Technically speaking, you will need to show your federal proof of vaccination on your way down and still have your ArriveCan app information filled out for your return. The only thing that is different is that for trips less than 72 hours you no longer need a negative PCR test.
Having said that, when I offered to show my federal proof of vaccination to the US border agent on the way in, they weren't particularly interested in seeing it. And on the way back, the Canadian border services agent did not look too interested in my ArriveCan app, since he knew that I was only gone less than 30 minutes. They were much more interested in the amount of goods I was bringing back, whether I had any firearms, alcohol, tobacco, cash over $10000, etc., like they normally do.