Operating system identifiers, in general, should only ever be changed if they are actually misidentifying something. There's extensive experience with this in the context of Autoconf's
config.guess identifiers; they get used in all sorts of places that you wouldn't expect (e.g. system adminstration automation) and therefore it's impossible to tell what might break if you go along with a marketing rename.
For instance, if
cfg(target_os == "macos") code was getting built on FreeBSD, that could and should be fixed, but the behavior of
cfg(target_os == "solaris") must not be changed just because
SunOracle's marketing department decided not to call their operating system Solaris anymore.
Similarly, it might have been better for Rust to call OSX "osx" from day one (but then we would be obliged not to follow Apple's rename) but now we are stuck with "macos" and anyone who wants to do a port to the classic operating system is going to have to call it "macos_classic" or something like that.