POST /v1/artifacts/authorities/{run}/forget: the run was deleted.
The authority record, its grants and its tokens are removed - there is nothing left for them to authorize - and one row saying the run was deleted takes their place, so a result the person chose to keep can say a run that was deleted instead of printing an id that resolves to nothing.
held_authority is whether there was a record to remove. false is an ordinary answer, not a failure: a run that never started never had one, and neither does a run that reached this device by sync.