I work for a Fortune 100 company with thousands of IT people. I literally have no idea who I would talk to about this. Maybe there is a slack channel for that ;-)
I would start with your immediate boss and maybe his skip-level. For the skip-level come with a precise explanation of what Slack is doing well, what your experience with Slack seems to show for negative and some potential remedies :)
Well, one problem is that I don’t work for the IT department at my company. I’m not sure if my boss (or his boss or his boss’s boss) even knows what slack is.