I like Slack, and having a group communication tool has only increased my productivity.
How? - Having a place to easily search for issues others have had in the past. Sure, you can search emails or ask the same questions, but it's nice to search and find answers from other peoples conversations. - Integration for production alerting, customer feedback, and deployment pipelines. Instead of manually digging through several different web UIs or using a bunch of different CLIs, I can just take a look at the corresponding channel. - Notifications _can be_ non-distracting. Being the correct channels, snoozing alerts, or just exiting slack when you need the deep focus headspace, is easy.
I specifically don't want to be called/texted/interrupted in person unless it's something severely important. 90% of the time its not. I would much rather ignore a slack notification than answer a meaningless phonecall, or politely tell someone to bug off in person.
I will say, the "always on" culture is hard to face when Slack/group messaging is a companies main point of communication. Luckily, I've only ever been in places where its been understood that you are only expected to be responsive during your work day.
But, I think the negative effects of the cost of distraction by slack are overblown. There are endless ways to be distracted these days, and Slack is not the worst thing out there. You can always just exit the program.