This is more than just a simple hillshade. An out-of-the-box hillshade in a GIS program just looks at the slope and aspect of each pixel, without considering the adjacent pixels. If you look closely at the long shadows cast by the mountains in Bishop-Taylor’s images, you’ll see that tall peaks are casting shadows that…

It’s not necessarily only showing legal immigration. It’s whatever the Census recorded, which may include illegal immigration. But you are correct that the Atlantic slave trade to the US was illegal after 1808.