With goldmasters, Apple still does work on the iOS and they do make changes from GM To public release. You are partly correct though, they will not make changes that effect apps, so that apps developed under the GM have no issues in public release. That is the point of the GM after all. But they usually remove debugging code, with iOS 7 they added new wallpapers in the public release, and in iOS 4 they removed GC from the 3GS. The changes they do make are usually low impact bug fixes that will not effect third party apps in any way. But they do make changes, that is for sure.
I've been using the iPhone betas for a long time now and I definitely know there have been more instances as well if you would like me to point them out.