That can't happen in any modern encrypted messenger. It does happen routinely with encrypted email.
pgp as a tool could integrate with that, but in practice fails for... many reasons, the above included. All the other key exchange / etc issues as well.
Even if secure email clients exist that always make right choices, because you can’t know what client all your recipients are using, all it takes is one person with a “bad” client (which, keep in mind, is a client that accurately implements the protocol but doesn’t enforce additional security rules on top) to ruin things.
https://cloud.google.com/blog/topics/threat-intelligence/rus...
Some Ukrainians may regret that the followed the Signal marketing. I have never heard of a real world exploit that has actually been used like that against gpg.
Yet one system is declared secure (Signal), the other is declared insecure. Despite the fact that the QR code issue happened in a war zone, whereas I have not heard of a similar PGP fail in the real world.
"Because linking an additional device typically requires scanning a quick-response (QR) code, threat actors have resorted to crafting malicious QR codes that, when scanned, will link a victim's account to an actor-controlled Signal instance."
This is a complete failure of the cryptosystem, worse than the issue of responding in plaintext. You can at least design an email client that simply refuses to send plaintext messages because PGP is modular.