It's not just Slack. It's our phones! Notifications are distraction engines. At the beginning of March, I went cold turkey on most social media (I still allow myself HN and a guitar player's forum), and completely cold turkey on using my phone for these things - I have to use an actual computer. I put Kindle where Facebook used to be on my phone, and always have a book on it that can be read in small bites for those…
And lock screen notifications are off for all apps. I only see a notification badge when I decide to pick up and unlock my phone.