Depends on D19012. Ref T13053. In D19012, I've changed "Thread-Topic" to always use PHIDs.
This change drops the selective on-object storage we have to track the original, human-readable title for objects.
Even if we end up backing out the "Thread-Topic" change, we'd be better off storing this in a table in the Mail app which just has <objectPHID, first subject we used when sending mail for that object>, since then we get the right behavior without needing every object to have this separate field.