> With exception of Swift, D, and bytecode based languages, the ABI is left to the vendors.

I don’t see exceptions there. Apple defines the ABI of Apple’s Swift’s implementation, Walter Bright (or his team) defines that of D, Python defines its ABI, etc.

If you were to write a Swift/D/etc compiler, you’re free to define your own ABI. Disadvantage is that you would give up linking with code compiled by the other compiler (workarounds such as pragmas, C++’s extern "C", are possible)