Are messages retained permanentlt, like in current PMs, and more common chat apps like FB, Discord, Slack, etc., or will they be more temporary like Steam or IRC, that only store until viewed and are gone?
This isn't true with regards to irc. There are many ways to store pms, like using a bouncer/znc.
The irc client irccloud does it all for you.
These are all third party solutions. Nothing is guaranteed to be temporary, but the IRC protocol at its core is solely concerned with getting the message across, not what happens with it afterwards.
The power of irc is that you can access it through hundreds of different clients and configure it how you like. IRC is capable of saving messages, is my point.
I guess I'm conflating clients with protocols. Most of these companies wrap up their chat in a proprietary client and don't really give you the freedom that IRC has with what happens with your conversations. Its very easy to run a bouncer for IRC but since it often is an extra step outside the default I think of more transient chat platforms as 'IRC-like' in a way that might be disingenuous to the protocol as it is used today.