We used to run an IRC server at work, and it was pretty exclusionary to non-engineers; setting up something that'd log backscroll required maintaining a vm somewhere, searching through history required another service, etc. If all slack does is chat (and video communication by way of an acquisiton), I'm happy letting them be 5 years old. I'd scrutinize a company that: 1. has no fallback mechanism of Slack is mission-critical and goes offline 2. isn't careful about including non-technical stakeholders in communications.

Edit: clarifying that I'm talking about my workplace, not a company I run.