Starting with version 9.0, you can find out what changed in the public API from the last major version release. From the releases page you can reach the source code for that release and the version of ApiCompatBaseline.txt in that tree will tell you what changed. A major version release is compared with the previous major version release (so 9.0.0 with 8.0.0) and a minor version release with the current major version release (so 9.1.1 with 9.0.0).