I can't speak for AIM, but I've never assumed IRC was ephemeral. Most IRC servers had and have numerous people idling in the channel with their client just archiving everything that happened. Many of those archives ended up published on the internet and that usually surprised no one.
Discord is even worse because it does the archiving for you and grants new users immediate access to the entire archive.
As a rule, each and every recipient of every message that you send has the option to archive or forward that message. That's a simple fact of information flow that everyone in the modern world must wrap their heads around. In this case, when you send a message to Discord you're transitively sending it to everyone who is currently or ever will be on a given server. That has always been true, and that fact has been exploited ever since Discord came about.
All these researchers have done is expose the kind of archiving was always possible and always happening.