0.227 uses the C++17 language standard, which came out in 2017. Because MacPorts and Homebrew's compilers rely on the libstdc++ that ships with the OS rather than using the matching version from GCC or LLVM/Clang, that means that 10.14 is now basically the floor for building MAME on macOS.
Well that explains that. Guess I might as well just build and ship with 10.15 for now. That's where I'm stuck on my mbp, and about all I can do until I get an M1.
Thanks for helping with my ignorance RB