The most secure way of messaging is becoming thoroughly versed in infosec and only conversing with other deeply security minded individuals using E2E encrypted apps.
In lieu of that, use signal. Matrix if you want some more features. But never for a second assume that using a “secure” platform or service makes you secure. Security and privacy are a never ending dance. The most important part of which is to keep moving constantly. Understanding new threats, keeping a close eye on updates for your client of choice, and maintaining a constant background suspicion will ultimately have a much greater impact than which platform you use.
Snail-mailing a handwritten note to your friend in a secret code that only the both of you know.
that doesn’t hide metadata, the postal service can track it and you can trace back both the handwriting and reverse engineer the code. Someone can intercept the mail from many stations on the way, including your own post box.
I’d say that many messaging services offer more protection than that method.
You could, however, use PGP with elliptic curve cryptography, and send that via packet radio or something similar on a frequency only you know. To an uninvited person this only looks like garbage data or noise.
