I don't understand how a multiline message in the Slack interface is any different from two single line messages. If the issue is you just want to type them at once and have them sent in one go, I agree with you, but clients for IRC definitely have had that feature. IRC also supports formatting like bold and italic: if you want the interface for that to be markdown, that sounds like a trivial addition to any standard IRC client.

But everyone in the team would have to set up their client. Not everyone would want to setup their client the same way (understandably). And not everyone is interested enough in setting up an IRC client, especially the "not technical enough people".

IRC –the protocol– isn't made for that. IRC clients can, but don't.