Windows 10 Mobile is dead. We've known this for some time, even before Joe Belfiore confirmed it by saying that new features aren't a focus. Microsoft only sells two Windows handsets in its online store at this time, and no major U.S. carriers sell any.
So it should come as no surprise then, that the company won't be releasing any new Windows 10 Mobile Insider Preview builds. The news was confirmed by Microsoft's Brandon LeBlanc:
Remember, while there was a Windows 10 Mobile Fall Creators Update, it wasn't from the Redstone 3 branch like it was for PCs. Mobile was actually an extension of Redstone 2 called feature2, and it only contained a handful of minor new features.
Most devices that have been upgraded to, or shipped with Windows 10 Mobile are still supported with cumulative updates though. Every month, Microsoft regularly patches phones that are on the Anniversary, Creators, and Fall Creators Updates. The only version that's actually not supported is the original version of the OS, 1511.
Again, today's news really shouldn't come as a shock. We really weren't expecting to see anything new on the Windows 10 Mobile side of things, but now it's just been confirmed.