I'm not sure how one of hottest COVID pop-culture prevention suggestions could be considered an underground concept at this point.
The misunderstanding might be in the magnitude of effect: Vitamin D moves the needle a little bit, but it's nowhere near a magic bullet against Vitamin D or a solution to the crisis (as many health and fitness influencers have tried to suggest).
Don't be so sure about that. When I first heard about the connection of D3 with the immune system, I asked a friend, who happens to be a cardiologist, about it. He said vitamin D prevents rickets, that's all. And an overdose produces arrhythmia, so be careful with the supplements. Indeed, that's the conventional wisdom found in text books.
According to the internet, a healthy immune system needs much higher levels of vitamin D than those necessary for a functioning calcium metabolism, and a D3 overdose is only bad if you have insufficient levels of vitamin K2. I don't know what to think about all of that. After all, my representative study with a sample size of one and no controls yielded no significant results.
Personally, I stopped having the vast majority of colds during the winter since starting D a decade ago. One or two colds since, and both were over in a day. Definitely a significant life improvement.
There are several dominant threads pushing back here on a professionally done study about the benefits. So no, the important information is not as widespread as you claim.
For example, most Drs now know about vitamin D. But do they know about A and K and the dosages and which minerals you should probably take with it when someone is extremely D deficient like in a lot of autoimmune patients? Probably not...
Do doctors know most patients with resections or digestive issues who are B12 deficient can absorb methylcobalamin sublingually? I've never met one that does.
I can go on. Most doctors understanding of B12 is still stuck in the 1970s along with most other aspects of nutrition.