Speaking of Build, Zac Bowden at Windows Central has a list of things to expect on Wednesday. Including this Windows Phone long shot:
I have in fact been hearing that internally Microsoft continues to develop CShell for Windows 10 Mobile in Redstone 3 builds. Whether that means we'll see it at Build, or at all, is another question. But we'll keep you posted.
I haven't researched this too much but my basic understanding of CShell is that it is the shell that is run on each device to give basic interface elements access to the core OS. Like the Finder process on macOS that gives you the Dock, menubar, etc. CShell is developed in silos for each device so specifically continuing to support Windows 10 Mobile wouldn't make sense if Microsoft didn't plan to continue to have Windows 10 devices (third-party or otherwise).
Again, as I said in my wishlist, I'd like to see Microsoft make a large commitment to Windows Phone so I'll join Zac in calling for this long shot.