Being offline doesn't help with distractions. People will send messages anyway and that red dot will be there. This is a long standing issue with Slack, you can

Being offline doesn't help with distractions. People will send messages anyway and that red dot will be there. This is a long standing issue with Slack, you can't silence it completely. Even muting channels doesn't help, you will get a notification or the red dot anyway. The only way is to go nuclear: uninstall it from your phone and close the desktop app. But that isn't doable in most companies.