On live feed shown on an incident, the trash can icon is always visible on all messages from all users, giving the impression anyone can delete anyone else's posts.
If, for example, an itil user tries to delete a previous post from an admin, a message appears to confirm deletion, and allows the post after confirmation.
After refreshing the parent incident form however, the live feed message appears again, so the delete operation did not actually occur.
After carefully considering the severity and frequency of the issue, and risk of attempting a fix, it has been decided to not address this issue in any current or future releases. We do not make this decision lightly, and we apologize for any inconvenience. If you have any questions regarding this problem, contact ServiceNow Customer Support.