The notification field disappears on the [sys_user] form when Notification Preference UI is installed and enabled.
All available releases.
OOB onLoad Client script on the [sys_user] table.
An OOB onLoad client script is responsible for hiding the notification field on the sys_user form. The script hides notification field when Notification Preference UI is installed and enabled.
Hide notification field - https://<instance-name>.service-now.com/nav_to.do?uri=sys_script_client.do?sys_id=f8824ed467d43200eae64ce742415a7d
You may disable the script by setting Active to false to show the notification field in the sys_user form.