I'm sure I've read much more about that, some 20 years ago, and after. Some small blips in the press, several fora. Sometimes even from people who claimed to be, or have been working as postman, and described what they had to look for (like house being renovated, new windows, nice garden, door, car, or in appartment houses clean floors, door mats, no trash, graffiti, what 'sort' of people) how to write it down in forms, and so on. Which annoyed them, because it was a hassle, unrelated to their job/task.
Anyways, they do have that data, gathered by whichever means, and sell them. As is obvious from their own sites.
It may be that they changed parts of that recently, because incompatible with EU-Law, DSGVO/GDPR, whatnot, but they did it.
[German] https://www.deutschepost.de/de/d/deutsche-post-direkt.html
https://www.spiegel.de/netzwelt/web/netzwelt-ticker-deutsche...
https://www.tagesschau.de/inland/faq-post-daten-101.html
https://www.deutschlandfunk.de/datenverkauf-an-cdu-und-fdp-d...
[English] https://www.deutschepost.com/en/business-customers/dialogue-...
---------------------- Letter is lost. Shrug.