Every great journey must have an end. As far as I understand Cirrus CI was struggling due to Github Actions eating their market. Cirrus CI was in my opinion much better than Github Actions but it is hard to compete with a bundled solution.
Defaulting to throw-away-VMs for everything is also the right choice for something where the threat model includes attackers submitting patches/PRs. I'll never understand why folks were ok with just container separation for that (and often have no separation in runners).