Slack's bait and switch
This is the same old model; give them something free, then monetize it.  Thankfully, the internet allows us to just go somewhere else or write your own. <a href="https://medium.com/@benhansen/lets-build-a-slack-clone-with-elixir-phoenix-and-react-part-1-project-setup-3252ae780a1" rel="nofollow">https://medium.com/@benhansen/lets-build-a-slack-clone-with-...</a>