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To keep, download, and play your uploaded songs at no extra cost, simply open a web browser, go to your Music Settings and click the “Keep my songs” button to direct us to save your music to the cloud. Otherwise your uploaded songs will be removed from your library on April 30, 2018.
Your Amazon Music digital purchases will continue to remain securely stored for playback and download -- no further action is required to retain those. These changes will not impact your ability to stream Prime Music or Amazon Music Unlimited."
I was using it as a place to backup music, though it had become increasingly difficult to upload more music thanks to Amazon's terrible Music App. Have a lot of this backupped on Google Music.