I'd like to take this moment to mention self-hosted, open source, and federated alternatives like XMPP and Matrix. I'd like to, but unfortunately I don't feel like I can in good faith. Matrix is woefully immature, and suffers from a lot of issues, but I think is closer to being a functional Slack/Discord alternative. XMPP is much more mature, and works very well for chat, but doesn't have a nice package that does all…
I can hardly see those as alternatives to Slack. Maybe https://mattermost.com/ is what you were thinking about?
Mattermost is certainly not what I meant. That's just trading one Slack for another.