OFFICIAL PAGE: Vocal Extractor
Vocal Extractor, which makes use of Deep Learning technologies, is available as a free download for iOS. The app will take a mono or stereo source file and return the vocal(s) and instrumental backing as two separate tracks. The developer is charging US$2.99 as an in-app purchase to unlock the ability to save your results. The results, even with mono source material, were quite impressive! The latest paid version of the app allows you to save your results as AAC or uncompressed WAV files and is now utilizing Spleeter for better results. The app is available in the Apple App Store. Details are available at the link above.
Screenshot courtesy of Christopher Kissel