You can send encrypted email. That's how email already works.

You can also send encrypted messages over any other medium. You don't need the messenger app to encrypt your messages for you.

One of the common arguments that PGP is bad is that it's "inevitable" that someone will send a message in cleartext, defeating the whole purpose of encrypting your messages. I don't understand this. The fact that this is possible to do is obviously an artifact of the idea that the user should be unable to tell whether the messages they send and receive are encrypted or not. Do the encryption and decryption yourself, and this is not a mistake it's possible to make. Don't confuse the encryption, which is something you do, with the delivery, which is something the channel does. The point of encryption is that the channel can't be trusted!