It’s a phrase used in a few different ways, but the main one is that if there’s a need to specify an exception, that indicates the existence of a general rule. Wikipedia gives the (good) example of a sign saying “No parking, Saturday 8.30am - 1.30am”. The fact that an exception has to be described for when parking is disallowed allows a driver to make the inference that parking is generally allowed.

I think EternalExplorer is saying that the degree to which people with masks stick out on the very rare occasions that you see them, the way they need to be specified as something other than the default, just makes it even more clear that the general case is that masks are now absent from public life in their city.