Honeypots are advertising that header as well nowadays:

https://www.shodan.io/search/report?query=x-clacks-overhead

Most of the non-honeypot results are for the Gargoyle Router Management interface exposed by Korea Telecom:

https://www.shodan.io/search/report?query=x-clacks-overhead+...

The results have increased significantly over time:

https://trends.shodan.io/search?query=x-clacks-overhead