Honestly the easiest way to test this is to make different 404 pages for app/ and pages/ routers, and then trigger them by requesting a page that doesn’t exist in each section.

First see if each router triggers its own not found fallback. Then delete the one of them.

If deleting the one from /pages still triggers the one from app, then you should be safe to delete it for good👍

If both routers are in use, /pages/404.tsx will still be used for routes handled by the Pages Router.

Actually I didn't find this to be the case in 15.1.6 . Unless something else is going on that I am not understanding.

it's depend on how you are doing routing

So i have a route /pages/[slug]/[...handle].tsx when I go to route /sdfg/dfgsdfgsdf it tries to fallback and getStaticProps. Because the resource does not exist getStaticProps returns { notFound: true } . Which I would expect to display the pages router 404, but it seems to return the app router 404.