As classic car insurance is non-NCB rated you won’t receive a discount for any NCB years you might have accumulated on a regular car.
However, if you have spare NCB to use (for example, if you are replacing a ‘regular’ car with a ‘classic’) some insurers will allow you to apply the NCB years to the classic policy, so it doesn’t become out of date or expire. The NCB years will sit static on the classic policy and can be called upon as and when you decide to buy a regular car. As a reminder, you can only use one set of NCB years on a single vehicle, so if you have multiple vehicles, storing spare NCB on a classic policy can be useful.