Previously images dragged into the Xcode asset catalogue could be named as follows
Yes, that's because those names were used, years ago, before there were asset catalogs, when the image files had to live in the app bundle itself. There had to be a way to let the runtime know how the image was to be used, and those naming conventions were the only way. Nowadays, though, that's not needed; the images can have any names, because they just go into slots of an Image Set. But Apple provides a shortcut so that you can easily switch from using the app bundle to using the asset catalog, thus coming into the modern world.
with iOS 13 and Xcode 11 we have the ability to add an alternate set of images for use in dark mode. Does anyone know of if there is a way to specify light / dark as part of the filename so that images automatically end up the light and dark slots
Yes, there is no such way. Apple long ago stopped adding naming conventions for further distinctions among image files in the app bundle itself, because now the asset catalog exists and it has slots. So obviously there is no app bundle naming convention for light/dark mode, which just started existing. There is no need for a shortcut because you were never using a naming convention to begin with. You don't need to come into the modern world because you're already in it. Just put the images in their slots. You can automate that process, but mere naming conventions is not the way.
