For making the newcomer can't read the former message ( I think this is crucial, cuz it's the CORE feature of WeChat groupchat, and in telegram, you can read the message before your joining, that nostr have already solved ):

When every time newcomer came in, the chatroom nesc changes ( automatic, UI senseless ), like HD seed or Merkle Tree, and so you can not read the message before, but former people can read the following message.