interwiki_forwarded_from was implemented in compat 51171ad05ca6af7f5ff , but that feature wasnt merged into the already existing core scripts/titletranslate.py. As a result, interwiki_forwarded_from is unimplemented in core.
Do we need this feature? We havent had any request for this feature to be implemented, so the answer is probably 'no'.
If not, we should remove interwiki_forwarded_from from the family files.