What is the documentation issue?

In the Custom Polyfills section of the Supported Browsers page for App Router, it still tells you to use a custom <App /> component, but that isn't a thing for the app router.

https://nextjs.org/docs/architecture/supported-browsers#custom-polyfills

Is there any context that might help us understand?

This discussion exists, but it was marked as complete when a solution was posted for the Pages router. Now that the App Router is a thing it's not very clear how you're supposed to set up polyfills without needing to sprinkle polyfill logic in every individual component that needs it, like those of @formatjs for instance.

Does the docs page already exist? Please link to it.

https://nextjs.org/docs/architecture/supported-browsers#custom-polyfills