alt Hacker News> No protocol in the world can force anyone to delete anything from their own device.
But they either do not sign the messages or allow repudiating the signatures. Matrix signs all events forever.
Matrix also makes the entire event history (minus message content depending on room configuration) available to servers on join, even if that server's users are not allowed to see it.
These are distinctions without a difference. Events replicated across several independent Matrix servers are not meaningfully different than events broadcast across independent clients in terms of observability or repudiation.