One of Android Q's key focuses so far has been privacy, bringing a big change to how apps access a phone's file system. In the most recent beta, Google implemented Scoped Storage, an important tenet of that privacy push, and it was immediately met with negative feedback and controversy.
Scoped Storage has good intentions: it's meant to allow apps to have its own storage area while preventing them from having access to the entire storage partition. When implemented, apps are placed in a sandbox and don't need special permissions to write to their own files. It also means other apps don't have direct access to that app's sandbox. However, apps still have access to files in shared collections including the default folders Photos, Videos, Music, and Downloads.
To prevent breaking apps that haven't implemented Scoped Storage (most of them at this point), Google also included a compatibility mode, which disables the restrictive storage permissions for Scoped Storage on apps installed before the update to Android Q beta 2, and for apps built for Android 9 Pie or older. The problems arise when someone uninstalls and reinstall an app — compatibility mode will be disabled for it.
In theory, this sounds like a great security feature for Android. The controversy stems from Scoped Storage breaking users' apps, changing how they use their phone, and an outcry from developers about not having enough time to update their apps before the final release of Android Q.
Now, Google has decided to pump the breaks on the new security feature by not enforcing the API in the next Android Q beta. Instead, there will be a more gradual transition to Scoped Storage, requiring apps to adapt to the new API over the next year in order to be ready for the release of Android R.