Windows Phone 8's, Windows 8's Days Are Numbered, Literally
It was not as disastrous as the Windows ME or Windows Vista (though some might argue that it was) but Windows 8 and even Windows 8.1 were met with far more resistance than Microsoft was expecting. That unwelcoming atmosphere was extended to Windows Phone 8, which was actually less problematic, forever cursing that "8" generation. Given those factors, it's somewhat surprising that Microsoft has continued supporting the OS this long though now it is finally giving out the dates when Windows 8/8.1 and Windows Phone 8.x can effectively retire.
There are various factors that lead to Windows 8 hate. On the desktop side, there was the forced touch-centric UI that gave desktop users no choice in the matter. Overall, however, it was the overall dearth of apps in Microsoft's then-nascent app store that caused both Windows 8 and especially Windows Phone 8 to rot. Things have gotten better since then, especially with Windows 10 but it was far too late for the previous gen OS.
Starting October 31 this year, new apps will no longer be accepted for both versions of Windows. Not that there many new apps rushing to get into nearly defunct platforms anyway. Developers, however, can continue updating existing apps as normal. But on July 1, 2019, Microsoft will also stop distributing app updates to Windows Phone 8.x or older. The deadline for Windows 8 and Windows 8.1 comes much later on July 1, 2023.
These dates mostly affect the population of apps on Microsoft's store more than support for the OS themselves. Not that there are many of them left anyway. Microsoft is encouraging developers to port their apps to the Universal Windows Platform (UWP), though the trend seems to be moving towards Progressive Web Apps (PWAs) instead.