And IRC history doesn't?

IRC doesn't even have the concept of message history. Any such thing is done by individual clients/bots.

Bringing up message history as a negative is just straight-up disingenuous with that in mind - the answer to both possibilities is the same: use a bot.

Whereas IRC history persists forever for free?

Oh, wait.

"Unless you are paying for it, Slack message history vanishes."

Same thing with IRC. With Slack, however, I can pay later, and still have access to the history. With IRC, once it's gone, it's gone.

  > Slack message history vanishes.

And?