Personally, I cannot make a comparison between Slack and IRC because I have never tried the former. From the discussion, it is clear that Slack has many features and attractions that are not available or harder to obtain with IRC.
However, for me personally, a major attraction of IRC is that I can easily use it from within Emacs, and automatically have the same Emacs key bindings available that I also use for navigating and editing files.
With IRC, I can therefore quickly copy and paste text between chats and programs, search the history etc., all in the existing Emacs session, using the same key bindings, with direct access to built-in features such as autocompletion, dynamic abbreviation and spell checking, and — importantly! — without switching applications.
There is an emacs Slack client, though I don't know if you'll find it a seamless migration or as polished as what you're used to with IRC: https://www.emacswiki.org/emacs/Slack
I think that was something available to Slack in the past when it had it's IRC gateway. Sadly they closed that down.
We all have our own ways of doing things and I personally prefer to have applications run independently so I like my Slack client and my IDE and terminal etc. etc. separate.
I've always found if one tool can do it all, it's generally a worse experience than dedicated software applications and doing a Cmd-TAB (I'm on a Mac) is just as simple as switching to another window in a single application
However, for me personally, a major attraction of IRC is that I can easily use it from within Emacs, and automatically have the same Emacs key bindings available that I also use for navigating and editing files.
With IRC, I can therefore quickly copy and paste text between chats and programs, search the history etc., all in the existing Emacs session, using the same key bindings, with direct access to built-in features such as autocompletion, dynamic abbreviation and spell checking, and — importantly! — without switching applications.