I have read the spec, used the service and also implemented my own clients before, that is why I’m so confused by what you’re saying, because this has not been my experience at all. If a user joins a channel, whether they are an admin or not, whether it is encrypted or not, then unless the channel is explicitly setup to only allow verified users to talk (not the default), my understanding is there is nothing preventing that new user from seeing all new messages in the chat.