Student groups are where students save important documentation belonging to courses: discussions etc.
When a course has passed the end-date of a term the student groups become invisible for the student.
Therefore, in order to give the student read-only access to previous groups it is not possible to use end-date for the term.
The result of this is that the next time the course is given, two courses with the same name will appear in the calendar list of courses, which will make it very easy to add a calendar event to an old (wrong) course.
I therefore suggest that Instructure makes student groups accessible, but read-only, after term end-date.