find packages not distributed by the current distro - for example after doing what you should never do: a dist upgrade from ubuntu to debian :-)

orphaned_packages.py

#! /usr/bin/env python3
# apt install python3-apt
import apt
cache = apt.Cache()
for pkg in cache:
    if cache[pkg.name].is_installed:
        continue
    if cache[pkg.name].candidate:
        continue
    print(pkg.name)