When a platform does not guarantees secrecy you just avoid holding private conversations on it. In my opinion, that's the spirit of IRC, a public place to discuss, because often channels are logged and the logs are accessible for anyone from the web. If not, just about anyone on a channel can do it without your consent. That's also true I think with other "secure" chat platforms because that's the nature of any-to-any chat protocols.

Then you use private messages as a way to talk to someone without disturbing, or away from the noise, of the channel - still not for really private conversations ; for this you don't need encryption, you need end-to-end encryption, but at this point you'd rather use a messaging platform rather than a chat platform.