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.