It seems a lot of pundits don't like popular chat platforms and don't want free software projects' communication to end up siloed behind one.
Drew DeVault wrote in 2015 to the effect: Please stop using Slack for open-source communities. It's non-free, bloated software, and joining just to read can be a hassle. Use IRC instead and provide a bouncer, pastebin, and file drop to members of your community. 1/4
https://drewdevault.com/2015/11/01/Please-stop-using-slack.html