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

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.