With masks being socially acceptable in most if not all the world now, I wonder if the lack of visible smile that causes will change the perception of a smile in places where people usually smile by default.
My expectation is that by years end, in Europe and North America at least, mask-wearers will be gently mocked everywhere outside of some large metropolitan areas (which have their own epidemiological concerns), and there won’t be any kind of long-term impact on facial expressions.