Earlier quoted context omitted.
depends on how they talk on slack and who talks to who. The company I'm working with at the moment has separate slack per team with each time being ~5 persons and then some people from other team on each slack who pass message.... the communication is not great and the collaboration between services isn't either
Entirely separate Slack instances? Not just separate channels?
They also have mails, Skype, Yammer, Ms Team, and a few different conferencing system on top of that.
it's a mess. For example nearly each team have their own Bitbuck, gitlab instance, github org , gitlab org or whatever VSTS use