Relay – a customized version of Mattermost – is another option, with a full two-way bridge into the Clojurians Slack and unlimited history: see this thread Relay Chat Instance for clojurians!
The Discord server has been around for ages but very few people prefer it to Slack which is why almost no one uses it (and from an admin/moderator and integration p.o.v. Discord seems a lot more bare-bones than either Slack or Relay/Mattermost).