If we’re deficient then it may well mean that our bodies aren’t being accorded the usual natural opportunities to manufacture it!
Therefore a deficiency here is an indicator of sedentary, indoor, face-in-a-screen lifestyles that risk all sorts of poor health conditions.
The solution to D deficiency is to manufacture it by touching grass, enjoying sunlight (unprotected!!!) and exercising—not in a gym—but in that extremely large room, lit by a bright, warm lamp that’s 93 million miles away.
We don't need to hypothesize about our bodies not getting the natural opportunities to manufacture it. We know we don't. Plenty of people live in places where the sunlight is insufficient for significant portions of the year to generate any vitamin D. Many others live in places where they may nominally be getting enough ultraviolet to make enough vitamin D, but have too much melanin in their skin for the ultraviolet conditions where they live.
This isn't all about "oooh sooo much indoors too much screen time stop sitting so much lol lol lol". You can go outside all day in the middle of winter at the 45 degree latitude, where a lot of people live, and you will generate zero vitamin D, no matter what you do, because it isn't the visible spectrum you need. You need something that isn't in the winter sunlight at all. AIUI, it's technically not the same part of the UV spectrum that causes sun burns, but you're at least on the right track if you think of it as if you can't burn (modulo melanin), you can't generate vitamin D at all.
Just taper into the sunlight exposure so you don't get burnt. Skin cancer is not really a price you want to pay for adequate Vitamin D levels.
Skin cancer is not something I anticipate, since I scrupulously avoid slathering carcinogens onto it. I protect with clothing, umbrellas, and old-fashioned hanging out in the shade [my skin tone is porcelain, minimal freckling, 100% Celtic edition]. 26 years in Phoenix metro with literally no burns and no cancer: homelessness and public transit benches notwithstanding.