XMPP was also years ahead and even federated! At my employer we self hosted a server after AIM and it was great. Lack of persistent history was just a plugin aw

XMPP was also years ahead and even federated! At my employer we self hosted a server after AIM and it was great. Lack of persistent history was just a plugin away. Adium was also a nicely polished client which also handled AIM, so the user transition was easy. Sadly I think Slack was largely marketing and timing: federation was nontrivial (unlike email).