How to make a drop-shadow for a Graphics3D lines and curves?

Question

I have a plane a one line parallel to the plane and I want to make a drop-shadow for this line onto the plane. I try Blur but I had have no succes and I did it using Inkscape. But, I like to know the easy solution.

(*x+y+z=2*)
Q = {-1, 1, 5}; P = {0, 0, 2}; normal = {1, 1, -1};
{u, v} = Take[Orthogonalize@HodgeDual[normal], 2];
st = Graphics3D[{
   First@ParametricPlot3D[P + t u + s v, {t, -1, 2}, {s, -3, 2.5},
     Mesh -> None, PlotStyle -> {Opacity[0.4], Gray}],
   First@ParametricPlot3D[Q - t (u + v), {t, -2, 1},
      PlotStyle -> {AbsoluteThickness[4], Orange, 
        Arrowheads[0.06]}] /. Line -> Arrow
   }, Boxed -> False, ViewPoint -> {0.97, 0.27, -3.2}, 
  ViewVertical -> {0.69, 0.7, -0.2}, ImageSize -> 400]

Answer 1

It is not easy for me to get such result in 3D. So I turn to 2D and use DropShadowing which introduce in 13.1 version.

Graphics[{{Opacity[.2], Parallelogram[{0, 0}, {{20, -4}, {5, 8}}]}, 
  DropShadowing[{-10, -60}, 4], AbsoluteThickness[5], Arrowheads[.1], 
  Orange, Arrow[{{6, 6}, {6, 6} + {12, 3}}]}, PlotRangePadding -> 1]