The post you're replying to isn't referring to "impermanence," it's referring to messages dropped in transit. IRC is an unreliable message transport even if all users are online and trying to listen to each other.

The most obvious version of this is a netsplit, but even single users can get isolated from the network and have all of their messages dropped without realizing it until minutes later (if ever). There's no cool name for that; it's just IRC being unreliable.

Maybe unreliability is part of the charm for you, but that bug is absolutely not a feature, and not intrinsic to being a "social protocol."

Certain use cases that Slack et al cater to, you might not get to choose impermanence. There might be compliance/regulatory issues that require you to have document retention policies.

But the lack of reliability I described a few comments up is not just a problem for 24/7 archiving. You will see it in everyday use: your connection will drop, it will take a while for a timeout to notice, and you will miss messages for that period.

Now in a real life situation you can mishear or fail to hear. But technology can make this better.