I am not sure iOS popularity would justify macOS as a server. What would be the use case? It's not app development; that is done just fine on the standard desktop macOS. It's not backend; that is done just fine on Linux servers, even in Swift if that's your thing.
Builds
You don't need any feature from the old server OS for this, though. You just need your workstation to be on a network.
A network connected to what
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?
Which Apple gladly sells their own Xcode Cloud infrastructure instead.
Yes. Somehow it’s actually even worse
> that was before iOS development took off
It was offered through the 2010s, iOS development had taken off by then, and the last release was in 2021.
In fact the number of unique apps available on IOS has declined since the 2010s