Am I missing something? Virtually the entire open source ecosystem resides on GitHub, an unprofitable walled corporation. Not having searchable logs could be an issue, but how that different than an irc channel?
https://botbot.me/freenode/docker/2017-08-03/?msg=89344351&p...
As for Github, their are absolutely concerns relating to their role in the open source ecosystem, and many people advocate for Gitlab and other solutions as a result. However, the concerns with Github are much more theoretical; the actual experience is quite good, whereas with Slack it's pretty mediocre. Plus, it's a lot easier to migrate a project away from github; because it's built on git you keep all your commit history, etc., and your issues and wiki are easily exportable. If I want to move my open source project from github to a self-hosted gitlab, it's the work of an afternoon.
None of that's true on slack.