Is it possible to convert the shadow of a element to a path and then flatten it in sketch 3?
To do what you're trying to do in Sketch 3 (3.1 if you just updated today), is by duplicating the gear path, nudging the bottom layer down, and changing the fill color.
Two layers, no shadows, but you get the desired effect.
I wish there was a way to do what you want to do though. That being said, the Sketch team is pretty receptive to feedback:
There's a plugin for creating long shadows.