You may explore Mattermost [1] if hosting on your own servers is a requirement. It is very easy to deploy and I found it very satisfactory, albeit testing with a relatively small team. I imagine scaling up would work. It ties in nicely with GitLab, which is another neat product for those who must not or wish not outsource information.

[1] https://about.mattermost.com/

Matrix [1], mentioned elsewhere in these comments, looks interesting too and the Riot [2] app is similarly styled to Slack.

[1] https://matrix.org/

[2] https://riot.im/app/#/room/#matrix:matrix.org

Matrix is basically open source Slack. It fixes a lot of the problems that IRC had, enabling continuous presence, central authentication, easy file transfer, bot integrations, and more. I really hope it catches on more.

Disclaimer: I help run a small Matrix network.

Yeah, the GitLab omnibus package comes with MatterMost bundled. It's a really nice solution for self-hosted chat.

> They too though made a Google-like transition and went from an open protocol (IRC/XMPP) to a closed/custom API. At least here, the server admin can enable those protocols if they want and the API seems to offer enough features that someone could probably integrate Slack into another IRC-like client.

As for the XMPP gateway it's just barely usable. From my personal experience it even tended to drop messages from time to time. It seems Slack added it just to claim XMPP compatibility. Just look at where is Slack placed in this ranking https://conversations.im/compliance/

Apparently they were built to mimic IRC but don't actually use it under the hood. But like you mentioned, they DO have some integration with IRC.