Slack. Apparently.
My current employer doesn't use Slack and instead uses XMPP and a web front-end called "Movim". I can tell you right away, it is shit. I'd gladly pay 10k/year for the privilege of using Slack, so please don't bash on it until you realise how bad the alternatives are.
Wait until you get Slack, then you can revise your statement.
I've used it for talking to third-party clients that hire us and I don't see why should I revise my statement. It just works, it does the job just fine. The only complaint is that the desktop client melts my battery but given that I run it in the browser it isn't that big of a deal.
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.
Maybe they'd like matrix?
Well we already had a proven self-hosted Slack alternative called Mattermost that pretty much everyone enjoyed. The push for XMPP is because some of those fanatics live in a parallel world where everyone is using XMPP and they'd be able to talk to people on other servers with it. Of course reality is very different.
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this (and I feel bad for some of the people who actually wanted to get work done instead of fighting with broken and outdated tools).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