As reported by a community user https://github.com/modcluster/mod_cluster/discussions/574 the modcluster websites (except for ci.modcluster.io) were not accessible on Friday.

The root of the problem was that we were still using A records to point to GitHub Pages. We are using CloudFlare for the DNS server and Michal kindly setup a modcluster.io space for management. The configuration now looks like this:

All setup as DNS only i.e. not going through CF since this causes problem for Jenkins workers tunelling on the CI and not necessary for GH pages routing. 

While at it, there's always been a problem with having the website at the apex which has been fixed now too. Also we have removed the legacy names ('ci2' used in Jenkins migration); removed the wildcard * name (since Danger: Do not use wildcard DNS records (e.g. *.example.com) with GitHub Pages! A wildcard DNS record will allow anyone to host a GitHub Pages site at one of your subdomains.)  added www name, added routing for apex. The CNAME of the modcluster.io has been changed now to include the www subdomain, https://github.com/modcluster/modcluster.io/commit/3ffd77c35afc25308f6b89776e1efd302fbe8796 and now while accessing https://modcluster.io/ directly, there is a 302 redirect to include the subdomain. Any https://random.modcluster.io/ now aren't routed.