Starting with PIDI Planar Reflections 5 a new shader has been added that renders only the reflections and not the rest of the reflective surface. This is particular useful in cases where compositing is necessary.
First, set the Clear to Color flag to true, assign a Background color with an alpha value of zero:
Then assign the corresponding Reflection Only shader to the Reflection Caster:
For extra precision in the Legacy (Built-in) rendering pipeline, you can enable the use of Reflection Depth in both the Renderer and the Caster.