You can still cancel Amazon Cloud if you're within its 90-day free trial period and don't need it for anything else. And if you'd still like to keep using Plex Cloud despite this development, you can always move your files to the other services it supports, including Google Drive, OneDrive and DropBox.
Here's a copy of the email the company sent out to its users:
"A few weeks ago we shared with you that we've had challenges integrating Amazon Drive as a storage option for Plex Cloud. The team has worked tirelessly to address these issues, improve the scalability and performance of our infrastructure, and to expand storage options by introducing support for Google Drive, Dropbox, and Microsoft OneDrive, all of which are working great. Unfortunately, the challenges with Amazon Drive have proven insurmountable at this time, so we have decided to remove Amazon Drive as a storage option for Plex Cloud for the foreseeable future.
Current beta users with a linked Amazon Drive account will no longer be able to use Amazon with Plex Cloud after December 31st.
If you signed up for an Amazon Drive account specifically to use with Plex Cloud on or after our original announcement, you should still have time to cancel while you are in their 90-day free trial. We realize some of you have uploaded lots of media to Amazon Drive to work with Plex Cloud and the transition to another Cloud storage provider is easier said than done. This was a tough call for us to make, but a necessary one made with our users' best interests in mind. If you already have content on Amazon Drive, there's info on options for migrating data to a supported provider in our forum. We look forward to coming out of the beta with multiple popular storage options that provide a simple, seamless, and beautiful Plex experience."