It's been a few years since we synonymized macos to osx. At the time, we did this to distinguish incorrect uses of the tag from questions related to the old “Classic” Mac OS tagged under mac-os-classic.
Now, Apple has stopped calling their Mac operating system “Mac OS X” and “OS X”—it's “macOS” from macOS Sierra onwards. Therefore, we should reverse the synonym direction and point/merge all osx questions to macos.
Wikipedia has done the same, pointing OS X to macOS.
Note that this will not affect existing questions carrying a tag related to pre-macOS releases (e.g., osx-yosemite).