It worked as an accidental implementation detail in CPython from some other optimization, but it wasn't intentional at the time. Because it wasn't intentional and wasn't part of the spec, that code could be incompatible with other interpreters like pypy or jython.

See my comment and the linked email at https://github.com/ericvsmith/dataclasses?tab=readme-ov-file... for dataclasses and 3.6. I think it's still true.

The reason Guido didn't want 3.6 to guarantee dict ordering was to protect 3.5 projects from mysteriously failing when using code that implicitly relied on 3.6 behaviors (for example, cutting and pasting a snippet from StackOverflow).

He thought that one cycle of "no ordering assumptions" would give a smoother transition. All 3.6 implementations would have dict ordering, but it was safer to not have people rely on it right away.

pypy implemented naturally ordered dict before cpython did.

jython never released a P3 version so is irrelevant, ironpython has yet to progress beyond 3.4 so is also irrelevant.