I would absolutely love for someone to write up up-to-date instructions with the latest Xcode. I tried it (I tried both putting libstdc++ in Xcode as well as just referencing it in the project), but keeping getting a “‘locale’ file not found” error when compiling. Something in the vtk framework (specifically Headers/vtkdiy2/include/vtkdiy2/fmt/locale.h) is trying to #include <locale> and the system can’t find it.
According to what I’ve read, Xcode support for libstdc++ was deprecated years ago, and you should use libc++ instead. What are the odds of getting things update to use that library instead? I have no idea how different they are, but at least from Apple’s point of view, libstdc++ is past its prime.