> The fact that the standard doesn't specify a name mangling scheme leads to the completely predictable result that different implementations use different name mangling schemes.

The ABI mess predates the standard by years and if we look that far back the Annotated C++ Reference Manual included a scheme in its description of the language. Many compiler writers back then made the intentional choice to ignore it. The modern day ISO standard would not fare any better at pushing that onto unwilling compiler writers than it fared with the c++03 export feature.