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.