Not entirely true. Windows Phone 8 SDK ships with XNA 4.0 as a supported project type (includes templates too). That's certainly not an indication of support beyond this release, but it at least allows for backwards compatibility (for now).
That said, yes, XNA is dead for the future. Shawn Hargreaves is on the Windows Phone team now and others from the XNA team have moved on or quit Microsoft entirely (like Nick Gravelyn). Such a good set of APIs gone for good is sad to see.
I highly suggest following the MonoGame project and any other future frameworks that use SharpDX.