Nick Statt, writing at The Verge:
We’ve known since last year that Amazon Music was planning to shut down its dedicated cloud music locker. Now, we have a date for when that process will begin. In an email to Amazon Music users, the company says uploaded songs will be removed from a user’s library on April 30th, 2018. You can however keep any music in the cloud by proactively going to your Music Settings and clicking the “Keep my songs” button.