PGP

Good for protecting the message itself, but it does nothing for metadata. These days the metadata is what kills you. (Who talks to whom, when, and how often reveals enough to press the drone missile button, apparently.)

OpenPGP + SMTP as a starting point as email can reach many and Thunderbird makes OpenPGP happy clicky simple. The body includes instructions for the E2EE chat servers to use. Metadata trail ends there. Other circles of friends are joined in by other means. E2EE ejabberd (OTR+PGP+OMEMO) servers switched up after everyone is on the first outer layer of chat servers. Old servers are wiped. Of course all of this assumes nobody is silly enough to use a cell phone or all of this is futile. Everyone is using a laptop they paid cash for and a clean install of a generic minimal install of Linux and Coreboot running a script to make everything ephemeral and the OS nearly immutable. No security distros as they have all been infiltrated, just a vanilla simple Linux and a script.

what...?

PGP is not a very effective software suite for encrypting actual messages people want to send to other people, because it's a decades-old tool designed for email workflows that arguably never worked particularly well and is not well-suited to how people want to communicate today. Chat apps like Signal and WhatsApp that include end to end encryption as a matter of course are encrypting far more messages between humans than PGP for emails ever did.

that would be illegal in this hypothetical future

comms, travel, banking. Exactly why all the privatization of this stuff has top priority for the current administration. They're making their own government.