You can actually notify castrated boars as you can and should notify all pigs born and reared.(for traceability) What you cannot do is register any pig which does not come up to breed standard. There would obviously not be any point in reg. a castrated boar, but not sure that you couldn’t if you wanted to!
oh I was told by a commercial pig farmer who fattens 2500 pigs that it is illegal. I don’t understand why you would bother doing it to them when it is not neccessary? What are the reasons for doing it?