When you archive a class, we archive all class assignments. We also archive individual assignments for a student in that class if the student is no longer in any of your other active classes. If you created new active classes for the year/semester/quarter with the same students, we therefore do not archive their individual assignments created when they were in your previous class.
You can archive any remaining individual assignments. To archive an individual assignment, go to your active individual assignments page (https://commonlit.org/individual-assignments/in-progress), click the three stacked dots in the corner of the assignment you want to archive, and click "Archive Assignment.