Comment by achillean

Around 40,000 services on the Internet are currently including the header:

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

For some reason, a lot of honeypots are also using that header so I filtered those out. The number of services has slowly increased over time:

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