Starting in April, developers will only be able to edit the "What's New" text when submitting a new version of their app. In other words, any changes to the text will now be subject to Apple's standard App Store review process.
The requirement will extend to editing an app's support URL or marketing URL, according to Apple's announcement posted in iTunes Connect.
While this is a minor change, Apple evidently felt the need to have a little more control over information passing through the App Store without its approval. It's possible that a few developers were taking advantage of the flexibility for bad reasons, such as pointing support or marketing URLs to malicious websites.