I'm not suggesting that it's replacing every use-case for email. But likely 90+% of internal communication within companies that use it. I've experienced this both in my previous large corp (20K+) job as well as my startup (<10).

The main difference is that Slack lacks a unique global id, so you can't add a user in Figma based on it's Slack ID unless you have a Figma / Slack integration. Email still rules for these use-cases - which is exactly why open-standards (such as email, RSS, etc.) are better than proprietary APIs that repeat the same use-cases over and over.