this is issue is coming from the way @available clauses are handled. Using it will result in code generated that does something like
if(__isPlatformVersionAtLeast(...)) {...}
where __isPlatformVersionAtLeast is *statically linked* function in the main app.
I reiterate, the check for current OS version is built into the app itself. And what happens, essentially, that SDK version you built with impacts the "known" OS version, so if you build with "old" SDK, than, say, 11.0 will become, say, 10.16 (the largest OS version known to SDK used to build the app.)
Sure enough, when you build macos player "normaly" you get app bundle that unity have built, and sure enough this cannot be the "latest available sdk".
Well, not exactly "cannot": for example if i run your project on the latest development unity version it works fine (because we have updated the toolchaion internally to newer xcode).
But not all is lost! You can build your project with "Create Xcode Project" checked in build window, which will generate xcode project that you can build with newest xcode, and then everything will just work