Okay, interesting, thanks for looking this up!
If the domain is still getting paid for, I have no problem. Since we are pretty sure no one is paying for it anymore. If so, that would be really mysterious.
Hopefully they will reply soon. I used info@denic.de now and I got at least an automatic reply now.
As the former owner of a webhosting company, you'd be surprised at how often this kind of thing happens. There are sites that live on for years after people have died, it's kind of wild how long some things will keep auto-renewing on billing for, even to a dead guy's credit card.