In this question closed means can't be added to, such as the list of pronouns, I, you, etc. can't be added to, but nouns can; a hundred years ago there was no internet or a word for fractals.
So, is the list of -n't words closed? It seems to be as you can't just attach it to any old verb. hasn't is fine but workn't isn't. I have found many lists of -n't words but none that claim to be complete, only some that say they have the commonly used words.
In this question I used three such words without realising I'd used that many! can't (three times), hasn't (my example word) and isn't.