Since 18th December 2019, any person to have ever purchased a registration and never used, or have once retained off a vehicle they owned and held on DVLA retention document for a period of time and allowed the Certificate to lapse (Expire) and have failed to renew the document, the rights to which are now lost.
Under no circumstances can these now be renewed due to new legislation and following a 24 month period since DVLA first announced the plans to cease renewals.
However disappointing it may be to have "lost" the rights to a registration previously purchased, with the opportunity to renew the certificate now passed, fear not and do not dwell on what cannot be retrieved. CarReg still have millions available for low prices (from only £155*) try making your own here. Since some expired certificates we have seen in the past, lapsed more than 8 years ago, these would have previously cost £200 to renew (£25 per annum) so buying new is not an awful lot more.
Private Number Plates are cheap and easy to buy and new un-issued certificates can be assigned the same day upon receipt following your confirmation of purchase 4-5 days before.