I think the only good way to do this is off of the back of C++ Modules. Having it as a compiler flag means that all files in a project, including libraries, mus

I think the only good way to do this is off of the back of C++ Modules. Having it as a compiler flag means that all files in a project, including libraries, must be compliant. With C++ modules putting an end to the naive #include, translation units could specify the C++ specification they are compliant with, and hence gracefully remove or add new features on a translation-unit level.