Cyrus IMAP has been using skiplists for its storage for around 20 years IIRC.

https://www.cyrusimap.org/imap/concepts/deployment/databases...

I was very excited to come across this when I was working at fastmail. We used Cyrus from the start, and I loved seeing such a cool data structure getting utilised in practice.

https://fastmail.blog/open-technologies/why-we-contribute/