Once a user has denied access to the camera or photo library in a Messages App extension, how would the user be able to change this decision. I don't see the messages extension apps in the phone settings
Messages extension app settings
The permissions are shared by the parent iOS app. So it would show up under the Camera permissions in settings under the name of the parent/containing app. Even if you've created a standalone iMessage app (i.e. selected "iMessage Application" for the project template), there is an iOS app wrapping the extension (shown in the Targets list, and where you'd put your NSCameraUsageDescription Info.plist entry), and it's that app that would be listed in Camera privacy settings.
I've got a standalone app and it doesnt seem to be appearing under settings. Is there someting special I need to do to get it there?
For a standalone app, it would show up in the Privacy section under the appropriate section. I just tried this with an iMessage app that requested access to Calendar, and it showed up under Privacy > Calendar.