I was searching on "Google Scholar", a search engine for research databases, and came across a couple things I thought might be of interest to some of the people here.
First, I stumbled across something called "vitamin B12 deficiency neurological syndrome (VBDNS)". I have seen mention made of, and personally experienced, "brain fog", difficulty with recalling words, names, etc that are well-known to me, and similar symptoms indicating loss of cognitive functioning, but didn't realize that neuropsychological testing had demonstrated consistent loss of cognitive functioning in some people with B12 deficiency. VBDNS might be a productive search term for some of the people on this forum.
Second, I came across a quite recent article on assessment of B12 deficiency. I read some of it, but it will be a long time before I finish it. So even though I have not personally evaluated the article for accuracy, usefulness, etc., I thought I would post the link here in case others might be interested in the article. Please feel free to make your thoughts known if you find that the article is misleading or otherwise should not be trusted