Page Not Found Passthrough is the Drupal 8, 9, ∞ port of Redirect 404.

This module allows you to specify domains that should be attempted as a fallback to find content when a "404 page not found" error is encountered on your site.

This module is useful when you are doing a gradual migration to Drupal from another site (Drupal-based or not).

Let's say you have a legacy system which contains most of your web presence and decide to migrate to Drupal, starting with the home page, but didn't want to orphan the legacy site.