Amazon has updated a support page for its Music storage subscriptions recently, showing that it plans to retire the service. New subscriptions will be accepted until January 15, and you can also upgrade your current plan until then.
The change won't affect purchases from the Amazon MP3 store, or your Prime Music or Music Unlimited libraries. This is only regarding music that you've uploaded to Amazon's servers to be playable through the Music app.
If you're on the free plan that allows for 250 songs to be uploaded, you should be receiving an update to the Amazon Music app for PC and Mac that abandons the ability to upload new music. You'll be able to play back music you've already uploaded until January, 2019.
If you're a paid member for the service that allows for up to 250,000 songs, or if you sign up before January 15, you'll be able to continue to upload music, and you can also continue to renew your subscription for the foreseeable future. Naturally, if that plan expires, you won't be able to reactivate it. Once that plan expires, and songs over 250 will be removed, and the remaining 250 songs will be accessible for one year.