After Microsoft made a big deal about Windows Phone, their mobile OS is now dead. Microsoft ended support for Windows Phone 8.1, which means it's the end of the road for Windows Phone, period.
Windows Phone was launched back in 2010, and within 3 years it was the third most-used mobile OS in the world, but it simply didn't have the legs to begin to compete with iOS and Android. Apple and Google's mobile operating systems are now home to 99.6% of the mobile OS market.
Microsoft continued to push Windows Phone into budget smartphones running their new Windows 10 Mobile OS, but it has only received small updates since it was released, with Microsoft pivoting into the "intelligent cloud and intelligent edge". The Verge reports that Microsoft has halted the manufacturing of its own Lumia-branded smartphones, with a rumor suggesting that Microsoft will just maintain Windows 10 Mobile support through to 2018.