This shouldn't cause anyone any problems, but we're looking to put out an update that will remove this warning either this week or next (iOS also requires some changes for iOS 14.5 so we want to get those done at the same time).
The requestLegacyExternalStorage flag is currently present in all AppGameKit apps since it allowed us to continue supporting deprecated functionality in Android 10 to read and write to the device's storage (called external storage from the app's point of view). Android 11 has now deprecated this flag as well.
The presence of this flag does not mean the app is accessing external storage just that it might be, so Google are detecting the flag and warning you that if you are actually accessing external storage then you need to follow some new rules. For this Google have introduced a new permission called MANAGE_EXTERNAL_STORAGE which grants access to all files in external storage in Android 11, this is different from the old WRITE_EXTERNAL_STORAGE permission which we currently use.
Since no AppGameKit apps currently request the MANAGE_EXTERNAL_STORAGE permission you will not be required to explain why they are using it. If you are using the WRITE_EXTERNAL_STORAGE permission in your app then this is ignored by Android 11 and you don't get any access to external storage.
If you want to remove the warning immediately edit the file at "media\data\android\sourceGoogle\AndroidManifest.xml" and remove the code that reads
android:requestLegacyExternalStorage="true"
be sure to keep the closing angle bracket > at the end of the line. Then export as normal.
To access images on the user's device use ShowChooseImageScreen() instead which will use Android approved methods for a user to select an image.