well, at least the issue says, they can be removed from the easily visible header, so they could also be encrypted.

but smtp will know anyway.

i think, this is a general issue with "who is messaging whom" - also signal, whatsapp, whatever have to deliver the message and cannot know this afaik when these delivery data are end-to-end-encrypted. so this is not only an email problem.

but with email, at least, there is no "global" server that sees everyhing (no, also not gmail outside us :) and can draw a huge social graph.