If you work on a part you have "in mind" and "in eye" what you are doing.The warning you would like to have would introduce a completely new aspect to freecad.
A milling path is created only when the part is ready constructed, and than comes the computer and makes additional cuttings...
For the warning you don't need to know the material, - only a note, that the user is reminded about it.
part "simple" and "inside part" with all all necessary dogboon cutouts: and both with filets instead