Depends on D19009. Ref T13053. For "Must Encrypt" mail, we must currently strip the "Thread-Topic" header because it sometimes contains sensitive information about the object.
I don't actually know if this header is useful or anyting uses it. My understanding is that it's an Outlook/Exchange thing, but we also implement "Thread-Index" which I think is what Outlook/Exchange actually look at. This header may have done something before we implemented "Thread-Index", or maybe never done anything. Or maybe older versions of Excel/Outlook did something with it and newer versions don't, or do less. So it's possible that an even better fix here would be to simply remove this, but I wasn't able to convince myself of that after Googling for 10 minutes and I don't think it's worth hours of installing Exchange/Outlook to figure out. Instead, I'm just trying to simplify our handling of this header for now, and maybe some day we'll learn more about Exchange/Outlook and can remove it.
In a number of cases we already use the object monogram or PHID as a "Thread-Topic" without users ever complaining, so I think that if this header is useful it probably isn't shown to users, or isn't shown very often (e.g., only in a specific "conversation" sub-view?). Just use the object PHID (which should be unique and stable) as a thread-topic, everywhere, automatically.
Then allow this header through for "Must Encrypt" mail.