A big red X over the data not to be used leaves absolutely no room for interpretation: DO NOT USE THIS DATA. Crude? Yes. But absolutely in fashion with even cruder systems like a red flag as in mechanical instruments. After all it is an aircraft, not an i-pad...
You get an X over the speed tape, the autopilot disconnects and all you have to do is follow the procedures for UAS. Of course, if you don't it gets tricky. Do you really need to know why exactly the speed is not available? To what gain?