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The emacs web-site https://www.gnu.org/software/emacs/download.html
has a heading "macOS" and, later, "Using MacPorts:"
Thanks for reporting this. Would someone change the former to MacOS?
If the emacs project bothers enough to provide code for users of macOS then maybe it should use the labels macOS users expect to find. Using MacOS suggests that the code does not support the most recent versions of the OS, which is not the case.
Apple does not care how you label things. Users, who sometimes do not have the choice, do.
Besides, all this has little to do with English language conventions.