What's the alternative if you want universal way to communicate? That's like saying to stop using voice calling. Anything else requires everyone to agree to use a specific client, such as Signal, it's not going to ever happen.
No one would want to do it but you can encrypt messages over any platform using GPG and then send it to base64 or xxd and the text looks like this and the person would have to know the password to decrypt it, they could write something but I have not seen it yet.
echo "Hello This is a Secret Message" | gpg -c --pinentry-mode loopback | base64 | xxd -p
Enter passphrase:
6a41304543514d435464436469574d2b4955762f306c49427a564d504b2b
6843776c30446c4e44784b754f57537339424c536e49784e5637556b766a
4a4a52683866375469574e383037796d0a50357542586a2b443966675257
616d514262315078754c744e367471307176414a6f326237544259636a7a
632b5a346a774146582f5050710a
echo "6a41304543514d435464436469574d2b4955762f306c49427a564d504b2b
6843776c30446c4e44784b754f57537339424c536e49784e5637556b766a
4a4a52683866375469574e383037796d0a50357542586a2b443966675257
616d514262315078754c744e367471307176414a6f326237544259636a7a
632b5a346a774146582f5050710a" | xxd -r -p | base64 -d | gpg -d
gpg: AES256.CFB encrypted data
gpg: encrypted with 1 passphrase
Hello This is a Secret Message
This way you can send anything over any platform that claims to be encrypted or not, but no one would be able to easily determine what you are saying.