Why isn't it Mac OS? It's already used like this on wiki for release dates etc.
Topic on MediaWiki talk:Sidebar
Apple are referring to OS X as macOS starting with macOS Sierra.
Yes, as Garrett stated, unfortunately Apple have decided to change the name of OS X to macOS, to match their iOS branding. While it's not yet fully released, we will probably keep both names available for compatibility, and refer to them as in the sidebar - with the old name in brackets.
It's important to remember that while macOS == OS X, Mac OS != OS X. It is damn silly, but what can you do.
Ooooooh I didn't know they changed it again. DAMMIT!