IRC networks are distributed, so when one server dies you can connect to another.

So yes, IRC channels have better availability than Slack.

Generally yes, it's old and stable, and it's not centralized so not everyone goes down at once. When you host your own you often get to choose the downtime as well, updates get run at midnight when no one is working, not at 10AM on a work day.

A service the scale of slack also runs into a whole host of scaling issues that a little intranet IRC client will never have.