Fun story on this one. I looked to see if the new ForeignObjectRel class had all of the fields we were using from the old RelatedObject one, but parent_model has been renamed to related_model. But then I noticed that Django has been returning the new object as the related attribute since 1.8. The reason this didn't break anything for us is because...we don't use this module at all.

It was copied into edx-platform along with the rest of the django-cache-toolbox package (which was added primarily for its cached authentication middleware, which we still use). An attempt was made to use cache_relation on profile objects, but this was disabled within 2 weeks becase "we ran into issues"

Nevertheless, the relation.py module lived on. In the 5 years and 9 months since then, the cache_toolbox directory moved from the root of edx-platform to lib, then lms/lib, then common/lib, then common/djangoapps, then openedx/core/djangoapps. It was also cleansed of PEP 8 violations and had its imports reordered to satisfy isort.

Although the original django-cache-toolbox repo has been unmaintained for years, there is a somewhat active fork still making releases to PyPI which has actually adapted to the field rename. But I think we can just let go of this module until we actually have a need for it again.