Preparations

1. go to the IDE
2. open the core/units/users/users_event_handler.php file for editing
3. add the trigger_error('sample error', E_USER_ERROR); code after parent::SetCustomQuery($event); in the \UsersEventHandler::SetCustomQuery function
4. save changes
5. open the core/units/logs/system_logs/system_log_eh.php file for editing
6. add this code and save changes:

protected function OnBeforeItemDelete(kEvent $event)
{
    parent::OnBeforeItemDelete($event);
    die('never executed');
}

Testing

1. login to the Admin Console
2. go to the User Management → Administrators section several times to generate several (> 3 records in the System Log)
3. in the database adjust SystemLog.LogTimestamp column for one of the records to be 6 months old
4. go to the Logs & Reports → System Log section
5. remember records in there and their LogTimestamp column values
6. go to the Configuration → Website → Scheduled Tasks section
7. run the rotate_system_logs scheduled task
8. confirm, that above added die(...) code wasn't executed
9. go to the Logs & Reports → System Log section
10. confirm, that records older than 1 month were deleted
11. select a few (not all) records > press Delete button > confirm record removal
12. confirm, that above added die(...) code wasn't executed
13. confirm, that selected records were removed
14. press Delete All button > confirm record removal
15. confirm, that above added die(...) code wasn't executed
16. confirm, that all records were removed