Please don't ask people to run non-free software to participate in your open source community. Many open source / free software enthusiasts are reluctant to use them, or flat out boycott them. What's more, you are putting your worthy community somewhere out of your control if you do this.
Slack and Discord are not cool and do not make your project cool. They make your project unreachable, now and in the future (archives). You won't be able to trace the history of your project when you will want to do it.
What's more, Slack and Discord are hype now but will probably be old-fashioned in a few years and you don't know when they will disappear. Just pick something already old-fashioned today but that somehow does not die.
I agree with you. At the same time, people use GitHub and see no issue with it.
Probably because you're nowhere near as "locked-in" to GH. You can move your source code and all of its history to another hosting platform in minutes with git.
If you want to move your Slack/Discord community to another platform, there is no easy way to export the content AFAIK.
Ironically (and I am not defending them), Slack[0] is the only one of the three that has an official way to export all messages. GitHub does not, unless you're counting their API, but then maybe you want to consider Discord things like this[1]?
[0] https://slack.com/help/articles/201658943-Export-your-worksp...