Firstly, we all know portal management which is a model driven app currently. This is been available since the day portals are called ADX Studio. It is a place where we can able to configure the various setting of the portal. On the introduction of the PowerApp portals, it is been designed as a dedicated model driven app called Portal Management, this will allow us to do the same.
Introduction of Portal Studio is a WYSIWIG sort of approach, which is quite targeting the citizen developers. This make life easy for the citizen developers to make changes to the pages easily and also able to visualise the change in the preview panel. This make things easy to maintain.
https://docs.microsoft.com/en-us/power-apps/maker/portals/portal-designer-anatomy
But if you think deeply this feature is also available when ADX studio is around. It will be available as a add-on on the browser while accessing the ADX portal. It looks something like this, kind of a nostalgic moment though.
My Trial 