*UPDATE* Sketch version 44.1 fixes this issue, pretty much negating the need for this plugin:

To the surprise and dismay of many, Sketch 44 removed the previously available ‘feature’ that allowed users to place a large artboard behind multiple other artboards and export a composite image of the entire set.

I hope they do restore this feature. In the meantime,  David Williames was kind enough to create a plugin that replicates the behavior of this deprecated feature.

“Export all selected artboards — even when there are artboards overlapping each other, with a single action.

This mimics the behaviour that occurred before Sketch 44. It does this by creating a temporarily slice around each artboard, and copies the same export options from the artboard being sliced.”