I must admit I have never noticed it as a regulation but I remember the common practice being bare earth then everybody started to sleeve it at about the 14th . I understood that it was primarily for identification but it was at least considered to be a more appropriate method because it added "some sort of minor insulation" properties too.
So far as I am aware, there is no regulation insisting on the earth wire being sleeved. It is obvious which is the earth wire, so no need to identify it. The one good reason to sleeve is to reduce chances of the earth wire making contact with the live parts.