Just saw this video on making private categories for a group of users:
The question came up in our community. As the admin of our self-hosted instance, can I create a private category for a group, and offer them complete privacy so that I am not able to see what they post? That group would have its own admin/moderators for that private category.
No, the only thing I am aware of to prevent admins from reading content on the site is to encrypt PMs with Discourse Encrypt (for Private Messages). Otherwise, site admins are able to access the site’s database, so there would be no point in attempting to hide content from them.