It'll go back into the bank of registration numbers. Unless someone wants to buy it from DVLA, that's where it will stay. It just won't show up on any tax, insurance or mot databases until it is assigned to a vehicle. Same as any registration you can find on here. https://dvlaregistrations.dvla.gov.uk/Does that not mean that AB11CDE must be still recorded somewhere and 'remembered' for that specific car?
When you check those databases, you are looking for a specific registration attached to a vehicle. Not the registration on its own.