Plex originally launched using a preview version of the Google Cast SDK, which is the technology behind Chromecast. Since then, Google has switched to a release version of the SDK, which is more stable and secure. The new version is part of the Google Play Services. The original preview version of the SDK is no longer supported.
Starting with version 3.4.5, the Plex for Android app has been updated to use the new Chromecast services and no longer uses the preview version.
Android devices which do not have Google Play Services will not be Cast Ready. This includes Amazon devices such as the Kindle, among others.