I wonder if Slack lets organizations do on-premises (for compliance purposes, as you mention) e.g., as GitHub Enterprise does (many companies don't want to keep their raw source code in someone else's cloud).
The enterprise version they note is secure enough but still discouraged because it’s less about security and more about perception. Why would customers want to use the Microsoft equivalent if even their employees don’t.
They don't have such a product yet
Kind of surprising, to be honest. Business communication could be more valuable than source code to a bad actor.