Slack is an all day company wide meeting with the whole company and no one brought an agenda. If you do not participate, decisions get made without you.
Slack prevents employees from having deep work sessions inside core business hours. Most employees with heavy slack flow just try to survive until the volume decreases.
Everyone knows that getting into a great deep work sessions is easier on the weekends or at nights, with no one bothering you.
Email is a mature communication medium, and is better than slack because of one key aspect.
In email, you can tell the whole company to bring a problem to a specific email inbox, which is monitored by three or four people. In that way, you can maintain confidentiality.
But in slack, their is no way to do this. If you have a slack channel, everyone who is in the channel can see what everyone else says.