Admittedly I'm only thinking of a single datapoint here, but in the case of Lavabit, the court order which was intended to grant metadata collection ability to the FBI specifically mentioned that "Subject" headers were to be stripped. At least in that case, the relevant government spooks weren't even trying to argue that "Subject" lines were in their purview.

They're headers, which categorically tend to be viewed as metadata even by technologists. You're right that they aren't necessary for routing, but this is not exactly the government being slippery. Even where encryption is used, metadata — including the subject — tends to be sent in the clear.

They're MUA headers, not MTA headers. That's an important distinction; those headers are generated directly by the end-users' systems, rather than by the systems responsible for routing the mail around.

Even that isn't a clear bright line. For example, the To: header also comes from the MUA, and it's crucial to the message's transit. (In fact, the To: header is very much analogous to the stuff written on the outside of an envelope.)

To: actually isn't used in routing. MTAs and MDAs route messages based on the target address that's communicated in the 'RCPT TO' command, ignoring whatever may be in the To/CC[/BCC] lines, should the MUA happen to generate those headers.

The set of metadata used in routing, aka the 'envelope', also contains a return path, which is (normally) used for things like submitting delivery failure reports.

No, the To: and From: fields you see in your MUA is the software equivalent of my saying over the phone, "Hey, this is Steve - is Dave there?" In my fairly educated opinion, of course.