It is, but the section just east of Elveden where there is a cycle path on the old road/alongside.
I knew about that section, but it seems pointless without the rest. And they even gave away part of the old road to a private estate, not that it would have been any good without the missing onward route.
Indeed, although a cut through would at least reduce the distance on the A11 in one direction. Since I was riding from Center Parcs Elveden, that stretch was useful for going to Thetford. As you can see, when me and my dad rode down to Cambridge, it was a substantial detour.