Well I received a reply from Greater Anglia. They seem to have grasped the concept now, though I’m not 100% happy with the language they used. What do others think?
To confirm, the 60+ Oyster Card is accepted for travel to Romford on Greater Anglia services, provided the train stops at this station. Additionally, the card is also valid (in a theoretical sense, as no trains currently stop there) on Greater Anglia services up to Harold Wood, but this is only the case if you purchase an extension ticket to your final destination from either Harold Wood or Boundary Zone 6. For travel on TfL or the Elizabeth Line services, the 60+ Oyster Card is valid all the way to Shenfield.
To be clear, it’s not theoretically valid, it IS VALID as long as an additional ticket is held.