I have been looking (in https://db.debian.org/machines.cgi) for a service or list of Debian packages without upstream (parent project).
Because I think there are a lot of deb packages which are alive in Debian but are dead outside.
The Ultimate Debian Database has an upstream version query which you can use to list all packages who either don’t have a watch file (to track upstream releases) or whose watch file results in an empty value (because upstream has gone, or an error occurred):
curl https://udd.debian.org/cgi-bin/upstream-status.json.cgi | jq '. | select(.["upstream-version"] == null) | .package'
This doesn’t quite answer your question, since it won’t find packages which do track upstream releases, but where upstream releases are unlikely to ever occur again (for example,
fyre, which I know has been abandoned for a long time). It will also find packages which are themselves unmaintained (they have an obsolete watch file, regardless of upstream’s status). But I think that this is the best you’ll get in an automated fashion.