unless you prevent new members from reading the chat history until given permission then they can already read everything before they are kicked out, and they can come back with a different account.

you also can not detect people acting in bad faith if all they do is read.

basically, you can't expect privacy if you don't limit members to people you know and trust. that goes for any group chat, encrypted or not.

i also doubt that discord chatlogs are encrypted on their servers.