Also, I forgot to mention - we did have an Slack clone called Mattermost running and the overall response from the dev team has been positive. The only reason it was shut down is because of some open-source fanatics who thought Mattermost wasn't open-source enough (apparently the fact that it's backed by a big company providing support is a sin) so we've switched to this XMPP abomination.
It needs to be company wide for you to understand the truckload of channels, the spam of notifications and gifs, the poor mobile app, the poor replacement of emails, etc.
I'm leaving soon anyway so I wish them the best of luck with this (and I feel bad for some of the people who actually wanted to get work done instead of fighting with broken and outdated tools).
Yes, we do have people wanting connectivity via XMPP, IRC, Discord, Slack, etc. There's "matterbridge" that offers those options for Mattermost: https://github.com/42wim/matterbridge