Permanent Cast button, new privacy icons are coming with update to YouTube's Android app
An APK signed by Google can be sideloaded, or you can wait for Google to push the update through Google Play. Those wanting to update their existing Android app with the APK will need to use a device powered by Android 4.0.3 or higher. We should let you know that this is version 10.19.53 of YouTube, and the file weighs in at 12.11MB.
More exciting is the code string discovered in a teardown. YouTube might be soon be offering something special for those users who employ the app as a music player. According to the discovered code, YouTube users will get the chance to save memory on their phone or tablet by storing only the audio portion of a music video. Currently, the video cannot be removed from a clip saved offline. There is no word on when or if this feature will be made available by Google.
source: APKMirror via AndroidPolice