> The OS you are running the build on and the Xcode version don't influence that

Usually it doesn't. But in this special case it does. I have set the flag you said. When C++17 wasn't in the picture, it's enough. But now we are talking about how to enable developer using the new language features. New C++ features typically need new runtime. But the old systems do have it. Then one of the difficulty is how to figure out which macOS versions have it, which doesn't.