Have you already tried to create a macOS document based app with SwiftUI and the new App protocol? Well, there are some problems to solve esp. if you want to use menus focused on the currently open document.
In the old app cycle with AppDelegate / SceneDelegate you still had to use the storyboard to define the menu items and link them to your methods in your document class derived from NSDocument. Enabling and disabling of the menu item according to the open documents was automatically managed by the frameworks.
That’s not the case in the new App protocol of SwiftUI anymore. Instead, you can define your menus and menu items quite nicely in your App struct conforming to the new App protocol but there is no automatic link to the current document in focus.