The answer above is not exactly correct. The “archive” feature is not meant to save students work before removing them from the license plan. When you remove a student from your license plan this student is removed from all your classes, no matter whether they are archived or not. And when a student is removed from a class all assignments of this student inside this class are removed as well.
The only way to save the assignment is to make a copy of this assignment inside your “My Spaces”: just find the assignment in your class, click the three dots button to open the menu and and then click “Copy to My Spaces”. Once you remove student from the license plan copy of this assignment in your “My Spaces” will remain intact.
See here how to copy a space to "My Spaces"