I think part of this is a cultural issue with older "traditional" companies. I have worked at such a company and, after introducing Slack, people used it in exactly the same way including cc'ing everyone at the end of the message and needlessly escalating every thread if it's not answered immediately.

The annoying thing with Slack is that it's impossible to mute individuals who use it disruptively so I either get all or none of these notifications. It's easy to change people's behaviour in smaller companies but when there are hundreds of people in the workspace it gets tricky.