Currently, there are no Microsoft APIs to back up Group Shared Mailbox, Shared Calendars, and Planner Tasks under Group & Teams. Therefore, we must add our Backup Admin to the Teams and delegate its permission to process backup. If the user wants it removed, we will not be able to back up Group Shared Mailbox, Shared Calendars, and Planner Tasks. However, other Shared Mailboxes will be backed up.
The system would automatically add the Backup Admin to the Teams Channel when it is removed. The Backup Admin gets re-added by the system automatically to avoid backup failures.
Note that this only applies when using the Global Admin method of connecting to your environment. When using Service Principal Authentication, no admin account would be required.