And if someone gets access to a channel that they shouldn't?
The best norms around slack channels is to have most channels be public, but have norms where not everyone is in every channel.
No channel will disclose private information. The only way to get that to work is to DM the agent directly or to use slash commands which are ephemeral / private.
I have lived in organizations where under 1% of messages were made on public channels. The experience of finding information in slack, or even knowing if there's a private channel where all of this is discussed, but where nobody invited you to, is not one I wish on anyone.