The only date deemed scarce is 1984 - the last year they made them and mostly issued in sets. Having said that, I don't think there are many people collecting them, so may be difficult finding a buyer even for a 1984.
They are still exchangeable at banks and the post office, although they hate them. Even though they weigh precisely half the 1p and so can be counted on the scales, for some reason they insist on counting them by hand!