Many things that target macOS and everything that targets iOS will have a CI running on macOS.
For better or worse, iOS is not a niche target.
For better or worse, iOS is not a niche target.
What's far worse than macOS not being a server OS is the hardware you're forced to contend with. Apple makes no server hardware. You end up with a disgusting hodgepodge of vendors making offerings that are largely redundancy in the way of massive falovers of mac minis. It's very unfortunate and makes self hosting or specifically (hardware or otherwise) needs extremely difficult.