A lot of cars that have been off the road for donkey's years pre-date the DVLA computerization. I bought a Capri that had been off the road for years, but first I called the DVLA to make sure they wouldn't try and fine me for non-taxation (doesn't make sense to sting the new owner, but you never know with the DVLA). The guy I spoke to said no, and that I should just tax or SORN immediately, as getting me on the log book would then cause it to get added to the computerized records.
I doubt they will bother updating old records of moribund cars - how many of these will have been scrapped without notifying the DVLA (before you had to be a registered dismantler) or will just rust in to the ground in a field somewhere?