Well, there are rules and regulations on privacy at your workplace.
For example: in my country, a folder named "Personal" in your email client is supposed to be off-limits for your employer.
Yes, it's stupid / ridiculous / impossible to to enforce, but still.
There are, as far as I can tell, no privacy guarantee in Slack *at all*. None.
And that bears repeating.
"I appreciate SDF but it's a general-purpose server and the name doesn't make it obvious that it's about art." - Eugen Rochko