I don’t have ADHD but I get very distracted by Slack. Here’s what worked for me.
1. Slack app uninstalled on my phone. If I need it for something, I install it, use it, then delete again.
2. Slack app on my laptop fully closed by default.
3. Set times (about 5 a day) to check in and respond to notifications and scan channels. When I was a senior manager with lots of actually important messages these blocks were about half an hour each (for a total of about 2.5 hours a day). These days I can get away with less than 10 minutes.
4. Block these times in your calendar. At the start of the day, block out the rest of the time without meetings etc as Deep Work. People will understand you’re not easily contactable.
6. Tell your close team mates/manager that if they ever need you urgently they can contact via Signal/WhatsApp. If anyone needs you and really can’t wait a few hours then they’ll ask your manager and be able to get in touch. If you’re really worried about being uncontactable then put your phone number in your Slack bio.
Using that, I went from being totally addicted to Slack to being able to be a productive worker again. Of course your mileage may vary.