This wasn't disseminated widely and it's not part of any formal preventative recommendation for doctors to implement.
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.
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.
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.