It must not be a restaurant or pub rule. Or they don't care. I've never gotten carded so long as my card rang through.
The rule applies to bars/restaurants too. I think there is a subtlety here. It's not that they can't sell, but rather "he had ID" is not a valid excuse to the liquor control board if it isn't the right kind. In fact, if the store wants to gamble with big fines and possible suspensions, I think they can legally sell to anyone who is in fact over 21, whether they can prove it or not. (The liquor board has to use testers who are in fact underage.)
(The 100% carding at large venues is different. Don't know if it is a blanket rule or part of their individually-negotiated charters.)
I've only seen the rule invoked once. A young-looking guy at a neighborhood liquor store who turned out to be 22 years old, at least according to whatever non-conforming id he had. The cashier was apologetic but said "I can't take that chance."
I wonder if it is going to be more of an issue now. We had a set of alcohol issues bundled on a ballot question at the recent election. It was mostly about how many licenses a business entity could have (a locals vs Total Wine fight) but updating the id rules was bundled with it. It did not pass. I'm sure it made more people aware of the rule.