I'd be pretty angry if I found out someone I chatted to on Signal was running a service to workaround my message expiry choice and archive my messages. And breaking that trust just to run it through an LLM?
Everything old is new again. This is basically the debate over IRC Bouncers all over again.
They might not be doing it on purpose, remember that microsoft windows "take a screenshot every few seconds and send it to an llm" thing?
fucken yep. don’t do this.
Be angry then, IM communication is not a contract with terms unilaterly decided by one party. Or put a confidential signature with every message, see how well that works for you