Before a site migration: python3 redirect_diff.py old-sitemap.xml https://new-site.com/sitemap.xml [--fail] lists every old path that has no match on the new site, i.e. every URL that will 404 unless you add a redirect. Accepts sitemap XML or a plain list of URLs. stdlib only; use --fail in CI.
Not SEO insurance: redirects keep links working; rankings depend on much more.
Disclosure: I build RoutePatch ($49), which turns the gap into a host-ready redirect map. This script is free (MIT).