And it's a close-walled garden, not anything like what the internet elders envisaged. IRC is a protocol. Slack is a product. AFAIK, you can't spin up your own slack service and run your own private slack (happy to be corrected)
We're currently considering adding some sort of backscroll support to IRC (maybe automatically setting up proxies for users), or installing Rocket Chat or HipChat on a local server.
While there's not yet C64 support, Mattermost connects to IRC, Slack, Gitter, Discord, Telegram and HipChat using Matterbridge (https://github.com/42wim/matterbridge).
In terms of features, the Mattermost community has added quite a few that aren't available in Slack--markdown support, multi-team accounts, threaded messaging, etc.: https://www.mattermost.org/what-slack-might-learn-from-its-o...
Mattermost deploys as a single Linux binary with MySQL or Postgres and you control everything.