It should be pretty easy to setup runners for most CI systems, somebody just has no need it.

In no particular order besides the likelihood I might personally run across a need I can think of...

• Gitlab CI
• Source Hut
• Cirrus CI
• Buddy
• Circle CI
• Travis
• Drone CI
• Jenkins
• Azure
• AWS Amplify

Feel free to comment on this issue with a request if you have a use for something else. At the very least I can give you some pointers for getting started.