I help hire people and consider strong opinions about which chat service is used a negative signal.

I don't have a strong opinion, but I do think using Slack shows that a company is vulnerable to chasing hype and eager to hop on bandwagons.

I don't see how you get that. Slack does tons of things IRC doesn't. IRC is not a valid replacement for the entirety of slack. To choose IRC means deciding that you don't need or want the rest of slack's features.

Exactly, how will our company be productive without emoji and embedded reaction gifs?

What about uploading files, video and voice chat, scrollable and searchable chat history, code snippet support? These are the reasons people use slack. To say IRC is a substitute means your company doesn't need to solve these problems (which is fine, they may not have them) or it means you're using a bunch of products together to solve them.

I don't get the stance that this is "hype" or a "bandwagon" at all.

What's a good way to get mobile notifications about an IRC message?

Install an IRC client on your phone?

If this is just going to be a game of "let's list every feature Slack has bolted on to their chatroom" then I don't care to play.

That was a serious question, I'm not sure there's need for the snark.

I'm glad I'm not the only one that feels this way and prefers IRC. :)