What is directory access permission? I recently got a Android 9 phone, and I noticed a new section under special access called "directory access". It lists no apps

This wasn't present on my old Android 8.1 phone, so I'm pretty sure it's new

What is it for?
The Android permissions model has been evolving with each release.  Starting in Android P, apps can request "directory access" to receive permission to access a scoped directory on the device.

For example, an app that requests the permission WRITE_EXTERNAL_STORAGE gets full access to external storage.

However, using scoped access, an app that takes and manages pictures might request access to the Pictures folder on your external storage device.  This allows you to ensure that the app in question can read/write data to Pictures and not say, Messages (or anything else in external storage for that matter).

The API is still fairly new so I'm not surprised that you don't see any apps currently using that feature.

Android has been fine tuning access rights for apps, allowed developers (and users!) greater control over who gets access to what.

You can read more about scoped access here: https://developer.android.com/training/articles/scoped-directory-access