> IRC is very much a centralised service even if you can host your own servers
IRC being an open protocol and having the option to run your own servers is what makes it decentralized. The various "major" IRC networks (EFnet, Freenode etc) are decentralized too, since they are isolated networks.
Slack offer neither of these - they own all servers, the protocol and controls the API (which is not the same as an open protocol).