What is an example of another Python implementation that has this property? Is there a concrete issue open somewhere that this is solving?

I am not unsympathetic to the idea of accommodating other implementations of Python, but this is very abstract and I think the assumption is probably that if we're explicitly testing for something and we don't say it's implementation-defined that it is part of the language spec.

If there's some evidence that stuff like this is intended to be implementation-defined always, or there's some concrete problem that we can solve (and possibly also add tests to avoid regressions), I'd be much more comfortable with something like this.