I used to particularly enjoy getting banned from subs for being mean - with a link to my comment being a reply to someone calling me an idiot and me telling them why they are wrong.

Except it is encrypted, and pretty secure. That’s not really related to the issue. Facebook complied with a subpoena as they are legally required to do so. Signal would have to do the same. The only difference there is that Signal doesn’t retain decryption keys for your data so subpoenaing them would be pretty pointless except to prove that some conversation happened.