Try the good old-fashioned man page for clang:
MACOSX_DEPLOYMENT_TARGET
If -mmacosx-version-min is unspecified, the default deployment target is read
from this environment variable. This option only affects Darwin targets.
Despite this, I will acknowledge that -macosx-version-min has been renamed to -macos-version-min (no x). So even the man page is a little out of date. Still, it should be authoritative.
Finally, you can use otool -l <path_to_binary> to confirm that the minimum OS is defined in the executable or library.
t_home
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