I think for .de domains, you are required to have your personal address in WHOIS if you are not a company.
First, I may be okay with it. And why would Posteo store WHOIS data? Unless they want to be a domain reseller, which is not what I asked them about.
Second, there are other TLDs.
I still insist that they do it because of lock-in and that they lie about it.
I think mailbox.org supports custom domains and is similar in other respects.
They should stop storing the mails themselves, they are full of private information. /s