There’s millions of examples of this. Everywhere there’s an oppressed minority group, that group eventually develops its own dialect that mutates quickly to make it easier to identify in-group members and to confuse authorities of the majority group.

That isn’t a secure comms strategy though, it’s just something that happens organically. All the oppressing group needs to do is recruit a defector from the oppressed group and they can understand everything.

Cryptography is how you do it. And cryptography exists and is easily accessible, unless you truly believe everything electronic has government spy tech in it, which is the realm of actual paranoia.