https://feature-sliced.design/docs/guides/tech/with-nextjs#app-router
You should still also add the pages folder to the root, because the App router is compatible with the Pages router.
Being compatible doesn't mean that you should add an empty folder. Real reason is probably this: https://t.me/feature_sliced/1/107414 as noted in https://github.com/yunglocokid/FSD-Pure-Next.js-Template