Shodan finds about 8,000 servers with the X-Clacks-Overhead HTTP header: https://www.shodan.io/search?query=X-Clacks-Overhead

Edit: I ran a query on HTTP Archive (which scans the top 1M sites) using Google BigQuery and got these results: https://docs.google.com/spreadsheets/d/1TYWRQWlWPuIWIj3dCHAk...