Text Classification : Classifying N classes vs rest of the classes
This is called an open-class text classification problem, it's used in particular for some author identification problems.
I don't have any recent pointers but from a quick search I found this article: https://www.aclweb.org/anthology/N16-1061.pdf
In the field of author classification there is a similar problem called author verification, which can be treated as a one-class classification problem. You could consider using it in this way:

one-class classification between &quot;known classes&quot; vs. others
regular classification between the known classes