http://u.qdnx.org/file/-20161019T095042-a8wr5ph.html

...yeah, or just IRC. There are mobile friendly clients, clients integrated for web browser use, multiple desktop varieties. They could even pre-configure and brand a "Creative Commons chat client" since it's all !floss.

IRC is excellent because it doesn't require a preregistered account, it is a common and well-spread means of communication, it can be integrated with !xmpp chatrooms if you control the server(s) etc. None of this is possible with Slack, at least not in any way that is future proof.