To me, the biggest issue with something like IRC is that any messaging system that only supports client-side search of message history feels anemic in 2017. It

To me, the biggest issue with something like IRC is that any messaging system that only supports client-side search of message history feels anemic in 2017. It needs to be seamless to use the system across multiple devices some of which will not be always online, which requires server-side history and search. Maybe you could cobble something together to achieve all of that using IRC, but even if you could it wouldn't be anywhere near as seamless as something like Slack.