Has there been any changes in the API that has not been communicated or have I missed something major here?
The classpath has not been set as depracted in the 6.3 documentation so I assume this is a mistake.
This looks like a doc Bug. For now, you can use  , search for "com.day.cq.wcm.api" and click on the name to check for deprecated/changed functions in 6.4. Also, log a daycare ticket so that we can fix the documentation issue.
 API Differences between AEM 6.3 and cq-quickstart-6.4.0-apis
The Page page is not there, linked from here for example:
Pls, verify that ALL javadoc pages are available.