You don't need any feature from the old server OS for this, though. You just need your workstation to be on a network.

My workstation is ill-suited to be a CI runner, even if it is networked

Right. A way to rephrase this would be: what feature from the old Mac Os X Server is missing from the current macOS desktop to make it possible?

A network connected to what

Which Apple gladly sells their own Xcode Cloud infrastructure instead.

Yes. Somehow it’s actually even worse