The goal is simple:

• the API is meant for plugins and extensions, while Maven "inner stuff" do have the static helper class
• having Service (instead of static class/methods) we are in greater control and are future proof
• for example (unsure how feasible this would be), we could test plugins on "spoofed OS"es?

I've enhanced the javadoc, but I wonder if it would make more sense to use proper enums, as we don't even define the values.

Yes, I intentionally left them out, as IMO this service would be most used for things like:

• print out some OS related stuff (like mvn -V does)
• the "is windows" stuff is hot and will be most used, as I envision, as many if not all plugins has IF/ELSE logic to perform hoops and loops

I just wanted to keep this OS stuff small, minimal. And once service, we can later extend it with new methods, as required or we spot some need.

Resolve #9877