Pass through requests to another (original older) site on 404 file not found

Hey someone has built exactly what i want, with even more advanced features:

https://www.drupal.org/project/redirect_404

It’s called “Redirects 404” on the project page and the pluralization is the hint of the advanced bit— it actually tries the paths on the fallback sites, and if they aren’t there it tries other fallback sites and ultimately can give you a normal not found message keeping the attempting user on the first site they tried (the new main site).

It hasn’t been upgraded to Drupal 8 yet though— it’s a Drupal 7 module.

Irrelevant resources: