This came up with a co-worker and I feel this is useful information for this group.
Number 1: There is no expectation of privacy when using your work-provided software. Assume everything you type will be archived and read.
Number 2: If your company uses a paid Slack plan, the workplace administrator can download the entirety of the workplace as a zip file. This includes all private messaging between parties on the workspace as well as channels and messages with parties on connected workspaces. Which brings me to…
Number 3: Deleted messages aren’t ever deleted. Most communication software doesn’t delete messages, they just tag them as “do not show” and leave the underlying data.
The reason Numbers 2 and 3 exist is both out of the duty to preserve information in the event of a lawsuit as well as to maintain compliance and allow financial companies to use the software (the SEC and FINRA have rules about archiving communications.)
Long story short, your Slack DMs can be read.
Do what you will with this information.