Not sure why this is being downvoted. Setting up IRC on a .onion isn't even difficult. Setting it up with SSL is only a touch harder.
It'd take more than 10 minutes simply to setup a server which ircd would run.
Then, of course, you'd need the time to setup all of the other services which Slack provides (i.e. durable file storage, integration with your applications and other services, etc.) and pick or develop clients for all of the platforms people use.
Then you have to actually support all of this: that's basic stuff like redundant servers and backups, account management, and the ostensible purpose for doing all of this: staffing your own security team at or better than Slack's level.
None of that can't be done and if you already have a good ops team it might even be worth doing. It's just way more than a 10 minute job even if you cut a bunch of corners.
* breach via a compromised (guessed) password * breach via a security issue in the service and/or the host
Hosting your own service may make sense depending on your knowledge and/or threat model, but it's not a silver bullet that solves all issues. Hosting on a .onion address buys you exactly nothing in terms of security - and in the case of a company/team chat server probably not even in terms of privacy.
I remember "SILC" which is not IRC, but if I remember correctly it had this. Mainly when you join a room, you should not get tabula rasa. You enter into some context, and if the last message was posted 4 days ago, your context starts 4 days ago, and the datestamps all reflect this.
There are other features of Slack that make it worth using, but this is the one thing I don't know if IRC can support at all, that I see every other serious chat system doing.
But the funny thing about all of this is there are already voice comms and chat comms clients out here in spades. The game industry created them. No need to go to IRC. Just use a more modern comms setup. I have a private mumble server myself.