Slack doesn't have public links and an archive of posts, it's a stream behind a login, which might go away at any time on an active project. It's a terrible idea for anything other than ephemeral communications which don't need to be logged or kept. Re the UI, for a chat app it's fine, most conversations around an open-source project should not be held in a chat format however, but in a format that respects participants not being in the same time zone or having the same time pressures (that means an async format, not a linear chat).

If you want to have an open process (for discussions around proposals, bugs etc), you need an open, linkable, archivable format, ideally hosted somewhere you control. That means mailing lists or a public html server, not a private chat service which may or may not be around, which may delete data or change policies at any time. For this reason Github isn't great either, but at least it has public links and you could scrape the data if required.