I think when the road tax exemption first came into being it was for "all cars over 25 years old" and was a rolling thing.
However, I think the chancellor soon realised too many cars would eventually become freeloaders and fixed the date for tax exemption to anything pre-1973.
Its unlikely to ever change now.
I was thinking of buying a Heritage rebodied MGB GT fitted with all new bits and just retain the registration document to get free road tax.
Until I drove one and that idea was soon dropped.