I go to work while the sun has yet to rise and when I come back home the sun is already away (at the peak of the winter, the sun is gone around 4pm). If I do go out for a walk, it's behind a heavy coat, scarf and hat. Often with sunglasses to protect against snow blindness and the wind.
This means that for about half of the year, my body does not see the sun. Glass windows will prevent vitamin D production so sitting by a window during the day will not help.
Between 70% and 97% of Canadians demonstrate vitamin D insufficiency. It's also important to highlight that people with darker skin need even more sun exposure to produce vitamin D as skin pigmentation negatively influences vitamin D synthesis.
Contrary to popular belief, vitamin D is a hormone. It impacts calcium absorption, which is the most known side effect. But it's way more than that. Many of the body's process simply cannot happen properly without vitamin D and the only way to get it naturally is from the sun.
One relevant symptoms these days is a weakening of the immune system. Many studies already show that vitamin D deficiency is one of the main factor behind the severity of covid infections.
Other symptoms are bone density loss, muscle pains, cancer risks, heart disease, nerve issues, blood pressure.
Another very important issue related to vitamin D is Seasonal Affective Disorder, which is similar (but different) to clinical depression.
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Edit:
I was wondering when Vitamin D supplements and enriched milk came around, since the current conversation in the context of this 1992 event.
There's a nice goldmine of information here: https://timelines.issarice.com/wiki/Timeline_of_vitamin_D
"1930 - Drug launch: Vitamin D prodrug dihydrotachysterol is developed as a method of stabilizing the triene structure of one of the photoisomers of vitamin D. This represents the oldest vitamin D analog."
"1952 - Product launch: Synthetic vitamin D2 and D3 compounds start being produced."