It seems in Unity 5, culled polygons no longer cast shadows, which is a problem for how I’ve set up a few scenes.
I can work around it, of course, but I’m not sure if it’s a design choice, or simply a bug.
And I must add, it makes more sense that they do cast shadows. If something is there, there should be shadows cast.
I submitted it as a bug report, because I hope it is a bug.
My apologies to the QA department in case it’s a design decision.
Case no: 644446
This is by design. If you want to have backfacing shadows, then a custom shader is the right way to go. Enabling backfacing for shadows by default would make rendering shadows by default slower.
It might make sense for us to add an option to the standard shader to support backfaces shadows specifically in the standard shader material UI.
I can work with that. I still have some trouble writing shaders that properly hook up to the new systems (the new cubemaps for example), but I’ll figure it out.
In any case, thank you for the clarification and my apologies to the QA department for a useless bug report 
Would it make sense from a performance standpoint to use separate low-poly shadowcasters instead of enabling backface shadowing?
I’m guessing it will depend on how complex the original geometry is. I made a variation on the Universal shader that cast shadows from backfaces and I’m going to use it selectively.