I may be alone in this, but I expect that everything I do on the Internet is done in public. Nothing new here.
So how do you send private messages? Carrier pigeon? Even Signal uses the internet, after all.
There's also Tor, which I use in combination with an obscure e-mail service that allows you to create accounts and login over Tor.
If you want to encrypt the actual email message, then I use GPG, but it isn't always practical.
Once keys have been exchanged, it switches to AES encryption with your peers, so carriers will only log a blob of encrypted text.
Then I got hooked.
I learned how to fold paper into its own envelope. I bought supple wax and sent a design for a custom wax seal stamp, so now I can mail a letter that is wax sealed domestically or internationally.
My friends love getting letters in the mail that's not spam. And a few even write back. One made his whole envelope out of duct tape.
We still call and text, of course.
And now I am practicing calligraphy in order to craft my own book, binding and all. I got very inspired when I saw the Book of Kells and Chester Beatty collection in Dublin.
Practically this has cost me (the fountain pen setup plus forever stamps) $100 USD. But that's because I bought a nice ink. So I think if people have a little bit of disposable income, and have Netflix to cut out of their life, could pick up old fashioned writing to catch up on life's details in private.
Heck, could even skip the writing and just buy the envelope and stamps, and type up a letter to print and mail!