Due to [this bug fix](https://github.com/vercel/next.js/pull/3849), we had to remove the `Router.onAppUpdated` hook. But the default functionality of this feature is still in effect.
In this hook, you can't wait for a network request or a promise to get resolved. And you can't block the page navigation. So, the things you can do is limited.
One real use of this hook is to persist your application state to local-storage before the page navigation. For that, you can use the [`window.onbeforeunload`](https://developer.mozilla.org/docs/Web/API/WindowEventHandlers/onbeforeunload) hook instead.