its sold more as a way to store and all conversations than the ability to be a messaging application.
the original pitch was to make all information, even private conversation of previous employees, searchable.
One somewhat paranoid department that was convinced they were being spied on (they weren’t; I saw the Slack admin dashboard and management was too cheap to pay for the retention and spying features) maintained the use of an ancient Jabber based group chat for their own internal communications.
The only restriction was if you went to China, you took a burner phone (one of the old company phones, usually) and weren’t supposed to ever use it again once you left. I think they just sold them to a liquidator.
Slack is also used because it allows to create persistent channels that are searchable. So they often end up being a knowledge base for the company.