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?

Morse code thrusts during sex.

act deaf, learn sign language, no one actually bothers to know sign language if they’re not deaf or near deaf people

I remember a meme years ago that described how the type, color, number, and arrangement of flowers in a bouquet could be used to convey complex messages. I’ll have to scroll through my hoard, but I’ll post it if I find it.

But, honestly, as long as there exists a code that is shared between dissenters, and ways to represent discrete elements of that code, communication itself is easy. You could encode anything in a medium that can represent a binary state, like lining up shiny pebbles on your windowsill: two close to each other for a binary 1, or a single one for binary 0. Or you could represent messages by how you hang your laundry to dry. Or embed it into how you play a musical instrument. You could hang Christmas ornaments in your window and use any obvious property to represent a message. You could use a rudimentary radio transmitter to send messages through radio noise. The options are limitless.

The difficult part is maintaining operational security. The limitations presented by human mental capabilities means that a very simple pre-shared secret must be used, which may be leaked or deduced. You have to know where to look, how to decode or decrypt the message, how to respond, and how to do all of that covertly. Prisoners have successfully used hand motions while cleaning their cell window to convey messages outside, apparently for years before authorities caught on.

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.

Pen and paper, even it’s outlawed people will always find a way to break the law. Of if you want something creative, through songs, poems or dance.

Reminds me of Arnis which the Spanish banned the practice so people created a dance around it.

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.

I can’t imagine how supernaturally thorough you would have to be to prevent written communication.

You can make pencils with charcoal, paper bark trees are common here.

Also pirate radio, hard but not impossible to conceal its location if you’ve got a good portable system.

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.

It’s really sad when no one is saying “just talk” 😂

I communicate by specially timed farting.

An underground movement forms that uses interpretive dance as a means of steganographic communication. The code is kept secret for plausible deniability. A parallel would be Capoeira, a combat technique developed by Afro-Brazilian slaves and disguised as a form of dance.

Return to Bronze Age, exchange clay tablets with secret messages in cuneiform. I can fashion a wedge-shaped stylus out of any old stick

Edit: better yet, if we don’t fire the tablets, we can destroy them quickly and easily whenever we need to or just recycle them. This was common for a lot of cuneiform tablets in ancient Mesopotamia, and much of what we have found in archaeology is stuff that was unintentionally fired because the building it was in burned down