And it never was an IRC client, there was just a somewhat limited bridge you could use to connect your IRC client into Slack network, but never vice-versa.

I'm well aware they are deprecated.

Besides taking my comment literally (I now have clarified), lets look at it from a higher level:

The concept of channels, usernames, private messaging in Slack is almost all directly inherited from IRC. There is some great new functionality on top of this core, IRC inspired experience.