In Service Portal, end-user can view a record on Form widget and this allows them to see the related lists at bottom of the widget. To avoid showing related lists for end-users below approach to be followed.
As an example, end users can see an incident record in form widget via 'sp' view in below URL format
https://instancename.service-now.com/sp?id=form&table=incident&sys_id=<sys_id of incident>&view=sp
Service portal Form widget uses 'sp' view to show the record details. Removing the related lists from this view will not show the related lists on Service portal view('sp' view).
- Open any incident record in service portal view as below in Platform
https://instancename.service-now.com/nav_to.do?uri=incident.do?sys_id=<sys_id of incident>%26sysparm_view=sp
- Configure > Related lists and remove the related lists from the 'selected' area of slush bucket and save it.
- You'll now see Incident without related lists for 'Service Portal'view
- Now open the incident in the service portal and the related list are no longer visible.