Microsoft held a Community Standup with Kevin Gallo today, where he discussed what's coming in the Windows 10 Fall Creators Update SDK, along with other things that are coming soon. When asked about ARM64 support for UWP, Gallo didn't have an answer, saying the following:
"We have not announced support for that right now. It's something we're looking at and evaluating, but I have nothing to share right now."
Obviously, there have been 64-bit ARM devices running Windows 10 Mobile since 2015 when Microsoft launched the Lumia 950 and 950 XL, but they all run a 32-bit version of the OS. At the company's Ignite conference last year, I asked when ARM64 support was coming, and was told that it's simply not necessary because there are no Windows phones with more than 4GB RAM.
But now we have Windows on ARM coming. There will be ARM-based PCs that can run x86 apps, and they will include an ARM64 flavor of the OS. Naturally, one might assume that that would mean that developers will finally be able to compile a UWP app in ARM64, as it would surely run better than an x86 app, or even an ARM32 app.
But with Windows on ARM PCs just a few months away, Microsoft won't confirm that support for ARM64 is even coming to the Windows 10 SDK. One would think that by now, the company would be accepting Windows Store submissions for ARM64 apps, so that it could do its best to build a diverse ecosystem of applications that can take full advantage of the software and hardware.
Of course, this isn't to say that support isn't coming, but it certainly doesn't appear to be in the near future.