Comment by progval
> 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.
> Matrix also makes the entire event history (minus message content depending on room configuration) available to servers on join
The important bit is the bit in brackets: as you say, historical message content is not shared if the room's is configured not to share history.