Christian described the listing criteria as follows: "Every day thousands of spam e-mails arrive on our e-mail servers, which have to be processed by our anti-spam system. If an email is recognized as spam, the IP address of the sender is recorded in a blacklist for a certain period of time in order to enable faster email processing and reduce the system load."
Today, May 25, 2020, he has announced that it is shutting down, due to concerns around GDPR and personal challenges brought on by the coronavirus pandemic.
He appears to be shutting it down in a graceful manner -- not "listing the world" as so many blacklists do as the end. This is good to see.
Note that in addition to the DNSBL dnsbl.inps.de, this also affects the DNSWL (whitelist) found at dnswl.inps.de. Both are ceasing.
If you use either the whitelist or blacklist in your email server config, you'll want to disable those checks as soon as possible.