As far as I know, Unity for example doesn´t offer stencil shadows at all, if you don´t buy the pro version for quite a bit more money than GS Pro.
Such a plugin is A LOT of work and that is actually the reason it was canceled, as time just keeps going on at a lot too much speed
There is no way I could imagine that he was "forced" to cancel it. Since I worked on shadows as well, I know that jcl is actually VERY supportive if it comes to developing features like this.
Other features of the pro version, which may make it worth to buy are for example the BSP, the terrain LOD, the automatic LOD generator of MED, the particle system which is a lot faster than with comm and of course the customizable start up window, which can even be comletely disabled as well as the file packing.
If you are seriously developing something with gamestudio those features are easily worth it, as they can save a lot of time.