@Gordon @Ratch - I don't know which way you searched, but the "registration database" will only show up active registrations.
I don't know that's entirely accurate? I use 'Check if a vehicle is taxed', a Government website, and have found an old H registration car (that is, the 1960s H) and it found it correctly, saying that there is no MOT and untaxed since 1982, which would pretty much tie in accurately to my knowledge. That said I tried a 1960s G registration and there was no information at all. Another K and R registration (1970s) were also correctly identified. So I think this particular website can query its database at far back as H though no presumably earlier. Whether or not those cars or plates still exist I don't know but the website also nots SORN and I can't beleive that the cars I have tried all exist still. That said I concur, the Maxi plate wasn't found on the database; but I wonder, could that be because it never existed?