Special notifiers used for scene deletion which lead to undo pushes after SCENE_OT_delete operator happening with scene still present in the mainfile. That was a reason why operator redo used to restore scene. It's not so obvious why special notifier type is needed for scene set and deletion -- it confuses undo system without having some obvious advantages. Using "direct" scene deletion and setting seems to be working fine so let's see if there'll be some issues with this.