So you made me curious and I googled. Not quite most everyone, it rather seems to be regional: under 20% in Northern Europe, 30-60% in rest of Europe, up to 80% in Middle East.
Source: https://eje.bioscientifica.com/view/journals/eje/180/4/EJE-1...
Source: https://eje.bioscientifica.com/view/journals/eje/180/4/EJE-1...
Historically this wasn't a problem since people with more melanin live in sunnier, more UV intense regions. In fact, pale skin may have been an adaptation as people moved north from the equator to combat the decrease in sun exposure/intensity. But people spend much less time outdoors today with AC and the internet, so vitamin D deficiency is especially prominent in dark skinned populations.
> The generally adequate vitamin D status in the Nordic countries is due to the use of cod liver oil and supplements (46) and vitamin D fortification, leading to a great improvement in Finland during the last decade (47).
Doesn't surprise me; here in the South we don't get deficient enough to cause obvious issues, so we don't pay attention to it.