The old folk saying is to take d-vitamin supplements in all months that has an R in them. Probably sound advice
This is just wrong.
Per capita vitamin D defficiency and insufficiency in the US are estimated at about 30% and 40%, respectively [0] compared to about 50% insufficiency worldwide [1].
Lack of exposure to sunlight is a major predictor of vitamin D insufficiency, even in western countries [1]. Sunlight produces more bioavailable vitamin D than dietary sources, and the recommended daily vitamin D can easily be obtained from sunlight alone, while it is hard to obtain from diet alone.
> Just a couple of slices of cheese per day and some milk is all the d-vit you need here apparently.
Daily intake of cheese [2] and milk [3] fortified with vitamin D at the recommended daily value is insufficient to make up for a lack of sun exposure.
[0] https://pubmed.ncbi.nlm.nih.gov/29644951/
[1] https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3356951/
https://www.sciencedaily.com/releases/2015/03/150317122458.h...
2. while there's plenty of evidence that vitamin D levels is associated with positive health outcomes, AFAIK evidence that vitamin D supplementation being associated with positive outcomes isn't very good.
edit:
3. I dug further and found there was a response to the study that you linked[1]. The response points out that the standard used by the original study (ie. supplementation levels that's needed for 97.5% of the population to achieve adequate serum levels of vitamin D) would cause people on the lower end to consume too much. Therefore the recommendation of 8000 IU (or any single number, really) is bad, and we should instead have different recommendations according to body weight:
>With this approach we estimated, for example, that doses of 1885, 2802 and 6235 IU per day are required for normal weight, overweight and obese individuals respectively to achieve natural 25(OH)D concentrations (defined as 58 to 171 nmol/L).
As such, even though there might be an error caused by "off by a factor of 10", everyone really shouldn't be upping their supplementation by a factor of 10.