This implies that all electronic communications are insecure.
While I like odd scenarios, this would be completely impossible to enforce, for one specific reason.
Anything can be a writing implement, it is simply not possible to ban writing as a concept.
You would need to ban litteracy to do that, and that would quickly ruin any country stupid enough to implement a rule like that.
I can absolutely see banning the printing press, xerox machine, and even a normal printer as they all quickly amplifies the message at a low cost, but normal pen/paper has too many uses and are too easy to make to effectivly ban.
This doesn’t speak directly to your question, but in the appendix of the Neal Stevenson book, Cryptonomicron, there are instructions on how to use a deck of cards (technically 2) as a one time pad for encrypting and decrypting messages. This could serve as the foundation for secret messaging using other media than paper.
Gratuitous acts of memery. Cats, Pikachu faces, swole doge and Cheems - you name it. Arcane to the uninitiated, ever so meaningful to the inner circle.
There are several registered cases of coded verbal languages emerging to enable elements of specific groups to communicate between each other without risk of being understood by outside parties.
Jamaican patoá is easy to identify as an example. The grammatical structure and speech speed creates a barrier for outsiders. Add a very plastic use of words and you have a very hard to follow “code”.
In the 60/70’s, in Greece, a private language emerged between gay men to enable those people, often persecuted, to communicate. It steadilly disappeared as social acceptance rose.
The cockney rimed slang is thought to have arised out of the need of the house staff to be able to freely speak between themselves in front of the employers with no concern from reprimands.
Then there is the prison and army slang.
Any more?
Briar. It was designed just for this. All traffic routes through Tor, it can also send messages using wifi, data, or bluetooth. There are no central servers to hack, seize, or takedown. Pretty much the entire internet would need to be killed off.
As for face-to-face communications - that’s been done for centuries to fight the good fight. No reason that can’t continue.
