are so-called because they contain a relatively small number of items to which no new items can normally be added. In English, auxiliary verbs, are a closed class: there are a small number of them, and the list very rarely changes. Contrastingly, an open class, e.g. nouns or verbs, receive (or lose) new items much more rapidly than closed-classes. Other examples of closed-classes are:
The closed classes of the English language, in use hundreds of years ago, would still be easily recognisable by users today.
See also Open class word