This is how all modern cryptography works. A deterministic cipher is functionally no different from pig Latin when it comes to actual security. An electronic solution like public key cryptography is infinitely more secure. If you’re especially paranoid you can generate the cryptotext locally and send it by email; that would be much safer than anything you could achieve by hand.
Odds are, it’s usually the opposite because they’re insecure and can’t handle when people do things differently from them.
If this isn’t projection then I don’t know what is.
It sounds like you ruined it for yourself. Judging from all of your replies and especially this one in particular, you seem to lack the ability to take basic accountability for your own actions. I would suggest reflecting on the cause-and-effect in the anecdotes you’ve shared and try to visualize how they might played out were the roles reversed and other people spoke to/treated you the way that you describe treating them. Even if you have some degree of sociopathy (I’m not a mental health professional by any means), you should still be able to reflect on these situations on an intellectual and objective level and consider that you might be the cause of these conflicts.
That’s assuming the key and message are entirely independent. If you or the recipient isthe type of person or doing the types of things that would attract surveillance from a nation state (because realistically that’s the one of the only scenarios where non-esoteric privacy practices might not cut it), it’s not unrealistic that they’d intercept both your digital and physical mail and would be able to correlate them. At least with public key encryption, the private key is never actually in transit.