That also goes both way: Pypy defaulted to ordered dicts a few years before cpython did.
The insertion order dict implementation actually comes from pypy
The insertion order dict implementation comes from Raymond Hettinger who is amongst other things a core CPython developer. pypy pulled the trigger on using it first (and probably has optimisations CPython doesn't). PHP also used it before CPython did, IIRC. And possibly Rust (as the third-party indexmap).
If they claim compatibility with only up to python3.6, they can have whatever order they choose.
The only issue with portability is that I think the main reason it was made a gaurantee is that cpython found the new, presumably optimized, implementation came with insertion order for free, so they went ahead and gauranteed it. But that might not be an optimal strategy in other areas, but they're forced to follow along anyways.
But actually moving cpython code to say ironpython should not be impacted, unless ironpython lies about it's compatibility