Please see the most recent updates in the "Where did the .com name go?" thread. Posts number 16 and 17.
That's because it's a FS2004 port-over Milton, FSX is backwards compatible with regards to the rain effect - IF - you have added the required textures either from FS2004 or via the fix you linked to. FSX does not have the required rain textures by default.
In addition, the ability to export the VC rain effect was removed in the FSX SDK. See this page for more info: https://fsfxpackages.com/en/blog/2014/08/15/vc-rain-effects/
Bottom line is you can get rain effects if (a) the model is a FS2004 port-over and (b) you have the required textures added to FSX so that you can see them but a pure FSX native aircraft cannot have VC rain effects.

That's because it's a FS2004 port-over Milton, FSX is backwards compatible with regards to the rain effect - IF - you have added the required textures either from FS2004 or via the fix you linked to. FSX does not have the required rain textures by default.
In addition, the ability to export the VC rain effect was removed in the FSX SDK. See this page for more info: https://fsfxpackages.com/en/blog/2014/08/15/vc-rain-effects/
Bottom line is you can get rain effects if (a) the model is a FS2004 port-over and (b) you have the required textures added to FSX so that you can see them but a pure FSX native aircraft cannot have VC rain effects.
Bottom line is you can get rain effects if (a) the model is a FS2004 port-over and (b) you have the required textures added to FSX so that you can see them but a pure FSX native aircraft cannot have VC rain effects.
There are several FSX planes that come with rain on windscreen effects, RealAir Duke and the Flysimware Mu are two that I know of.
Add to that Ant's Airplanes T28, another FSX native aircraft. As Nick pointed out, it just needs to be programmed into the model. There is an FPS hit which is why I think most developers don't include it.