Slack has the saving grace of optionally supporting an XMPP gateway, for those still committed to that dream. For now.
(I sincerely hope this comment isn't too prescient.)
(I sincerely hope this comment isn't too prescient.)
Slack is very much an organizational groupware product, with chat done well. Not the utopia federated XMPP promises, but frankly, XML just needs to die. (Which, not coincidentally, is part of what attracted me to work on the telehash specification.)