Should have just stuck with IRC :(
Slack IS IRC... just with new clothes on. :)
Not even close. Slack regularly screws up text, harfs on things, and does generally not-IRC things. Overall Slack's interface is terrible and does not seem to have been tested by anyone who has regularly used chat interfaces in the past.
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)
You're right. We were looking for an internal chat solution for our company. Currently we use IRC, but it would be nice to have an offline mechanism so we don't need to run IRC bouncers or CLI clients on our servers to receive messages when we're not in the office. We considered Slack, but decided it was too risky to host our internal communications on someone else's service. I'm waiting for the day Slack has a huge data breach.
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.
Mattermost might fit the bill. I haven't used it, personally.
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.