Discovered following the steps of
MDLQA-5176 when restoring the backup, after removing the source private and course legacy files.
When I restore the backup I get a "Some aliases included in the backup file could not be restored. The following list contains their expected location and the source file they were referring to at the original site" (attaching screenshot)
Also when I create the backup with an assignment (using references to images of private and legacy course) and a folder (same references) I get this notifyproblem message "File references to external contents included in backup package, they won't work on other sites." when the backup is completed.