Summary:
Activity column is blank in Historical Timesheets for Disabled Users. When a user's login is disabled, the Historical Timesheet Data should stay intact for reporting purpose. If this is shown blank then the issue might be related to Seat Assignment

Resolution :
This will happen when along with checking Disable Login, even the Seat Assignment and/or the Permissions are also unchecked. Once you put a check mark on the Disable Login for a user, there is no need to uncheck the Seat Assignment or the Permissions boxes for this user. The system automatically will free the license seat for this user. The Permissions and the Seat Assignment will ensure that data in the Historical Timesheets is showing all information and nothing is hidden or blanked out.