Yes and yes.

Both of those platforms have fairly low barriers to exit.

Moving off of github is a simple clone. Google Groups is archived, and there are many other alternatives.

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

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?