IRC is better by the fact its an open standard and you have your choice of tens of dozens of clients across any platform that connects to the net. If one does not like Discords or Slacks UI or the way you interact with it you are stuck with it , with IRC I can use GUI, CLI, TUI, text2speech, even Braille! Discord and Slack are closed locked down platforms that will not be around in 10 years whilst IRC will still be around 50 years from now.