First of all, this is a good occasion to say thanks to Kartik Agaram for his archive, which I consult regularly, whenever I have vague memories of some past discussion.
Next, IRC. There are communities that still use IRC to good effect, but I guess that anyone used to Slack will be disappointed. XMPP is one step up in features, but still quite minimalist. Which is fine with me, but it seems I am the exception. The next step up in my opinion is Matrix, and then we get to the Slack clones (Mattermost, Zulip, Rocket Chat, etc.). I'd say the first decision to make for anyone wanting to leave Slack is to define their preferences on this scale. And then consider the dependencies, in terms of (1) Who can mess with the community through the communication tool? (2) Who has an incentive to do so? (3) Who might do so involuntarily, e.g. for lack of means for maintenance?
It's on point (3) that IRC and XMPP shine. There are multiple options for both client and server software, all of which are manageable for people with moderate tech competence.