I'm not sure if there are any research showcasing the effects humanity has had in general due to low sun exposure. From all the benefits of Vitamin D and the recent human behavorial shift leading to low sun exposure (car travel, air conditioning, sunscreens even), there are bound to be new biological or psychological changes humanity is experiencing for the first time.
One study found a difference in mortality between the max-sun-exposure and min-sun-exposure cohorts: the second was twofold higher, which is comparable to the effect of cigarette smoking. https://pubmed.ncbi.nlm.nih.gov/24697969/ . I.e. avoiding sun exposure completely is as harmful as smoking cigarettes.
I've wondered for a while if the apparently higher cognitive performance and resulting societal wealth at higher latitudes might be some kind of second order side effect from long-term selection for lower sun exposure. We already know this vitamin D is almost certainly why Northern peoples evolved lighter skin.
(I realize this is a frought topic, so please hold the race science bullcrap replies or the over-reactions in the other direction. I am not a believer in hard biological determinism or "race science," but I also don't dismiss the existence of variations. As with everything else in population genetics and biology, any variations that do exist probably have more than one cause.)
If there's any truth to this, it might be further compounded as people with darker skin spend more time indoors in the modern world. If you have darker skin you need, as far as I know, more sun to make vitamin D, which normally is not a problem if you're outdoors near the equator. Maybe darker skinned people need to be taking more D supplements.
>car travel, air conditioning, sunscreens even
And even clothing.
What I find more plausible is that low sun exposure is one small contributor among many