The Official Feed Viewer Blog
Almost all of Feed Viewer users have upgraded their devices to Windows 10. Windows Phone 8.1 support ended on July 11, 2017. Windows 8.1 will reach end of mainstream support on January 9, 2018. Feed Viewer for Windows 8.1 and Windows Phone 8.1 will become unsupported on January 2018.
Also, the latest client for Windows 10 now requires at least Anniversary Update (version 1607, build 14393). Windows 10 RTM (version 1507, build 10240) support ended on May 9, 2017. Windows 10 November Update (version 1511, build 10586) support ended on October 10, 2017. The policy for the Windows 10 client is to map Microsoft support policy of Windows 10 versions. In some valid reasons, the support may be extended. Most of high-end Nokia Lumia devices will not be upgraded beyond Anniversary Update. There is no technical reason to cut the support for those devices yet.
Cutting the number of supported clients from 3 to 1 on Windows allows the expansion to another platform. Many work have been already done in this area. Follow this blog to receive the latest information about actual progress.