The subject line is message text! It's message! That's literally what it is! It's baffling (and revealing) to see email encryption advocates downplay the fact that subjects are exposed in plaintext. The retcon here is that subjects are somehow intended as the unencrypted, public part of the message, as if that was a normal feature of a sealed letter. Go find the last letter you mailed someone (or try to remember; I know it's been awhile) and look on the envelope for the part where you wrote what the message was about.