Same, actually... I'd probably just say "for macOS" at this point, since it is the current term from what I understand.
Aside: the project seems interesting enough, didn't see support for (icecast) streaming listed in the project though, which although less common today still exists.
That's how I read it, too! I got excited! "Finally," I thought "I can run WinAmp on my OS/2 Machine". Then I clicked to the repo and saw it was just a Mac thing.
macOS was never known as OS/X. It was formerly known as OS X and Mac OS X.
As a former OS/2 user, it really threw me off for a second.
Also, you're using a trademark ("Winamp") in the name. So, expect a C&D from Llama Group or whomever.
Aside from that, it looks really nice and well-designed! And it's in Swift and not some janky Electron app. Good job on those fronts!