Many popular ircd's support different IRCv3 specs. Freenode, Rizon, etc all support different specs/capabilities. You can see a list here: http://ircv3.net/support/networks.html
(Warning: bias/plug; I contribute to Lounge)
In terms of OSS alternative to slack, I recommend you check out Lounge, a self-hosted JavaScript web client (think: selfhosted IRC version of Slack, minus/plus a few features)