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
#
# requirements:
# apt install python3-apt

import apt

# as the script will list manual installed deb's,
# you can ignore them by adding the name to this list
IGNORE_PKG = ['check-mk-agent', 'yet-another-package-name']

cache = apt.Cache()
for pkg in cache:
    pv_downloadable = False
    if len(pkg.versions) > 0:
        for pv in pkg.versions:
            if pv.downloadable:
                pv_downloadable = True
                continue
    if not pv_downloadable:
        if pkg.name not in IGNORE_PKG:
            print(pkg.name)