Well as per the article (and my own experience), the free tier only gives you public channels. The paid tier gives you everything: public/private channels, group chats (called MPIMs), and one-to-one DMs.

So yes, it breaks "privacy" (not that you should expect privacy when using a work Slack account).

Admins can break DM privacy on most company accounts.

IIRC, you have to do something called a "compliance export," which just like any other compliance feature (SSO, HIPAA BAA, audit logs, etc.) usually requires the highest plan. It's designed to add some extra friction so admins can't just add themselves to a DM from the main UI like they could with a channel, but it is possible.

If you think anything you do on your work computer or saas accounts is private I’ve got a bridge to sell you.