In most places where I worked, I signed an explicit consent form stating that all company-provided means of communication are for work purposes only, and may be audited. I suppose it's required by law.

So my rule of thumb for workplace is: expect no privacy.

If you want to use work-provided email, slack, etc to discuss things which you'd be very uncomfortable discussing in your office in the open, especially in the presence of your bosses, don't. Find a different venue.