A student who at any time was removed from your course (and not re-enrolled) is an inactive student.
You can view submissions associated with inactive students by selecting inactive students from the top right of the Submissions/Students page. If you don't see this option, then your course doesn't have any inactive students.
If you re-enroll an inactive student, that student will no longer be considered inactive.
Note that you cannot upload new submissions associated with inactive students; you can only view existing submissions that were uploaded prior to the un-enrollment of the student.