Via Apple’s App Store Review Guidelines:
2.3.12 Apps must clearly describe new features and product changes in their “What’s New” text. Simple bug fixes, security updates, and performance improvements may rely on a generic description, but more significant changes must be listed in the notes.
Only 5 of the 12 apps my device has received updates for in the last week have had actual changes listed. And I still find when I launch apps that major changes are made (like the inclusion of ads) without informing me first so I can make a decision whether to update or not. I don’t think Apple is really cracking down on this.