@SunflowerSimmies Thanks for the links to the Unity forum; I'll take a look and see if I can find anything that might apply to Sims 3. This is outside my comfort zone, so I don't know whether I'll be able to figure out what's going on.
I know about the forums.thesims.com thread; if you read through the whole thing (don't do that, really), you'll notice I posted there too. The lack of dithering, which accounts for the color banding and pixellation, is a universal issue having to do with Windows 10 updates and recent graphics drivers. It's not necessarily related at all to the white lines—I don't have those lines, nor do most of the people posting about the dithering issue.
The white line along the mountains in Moonlight Falls is yet another probably separate issue, one that's been around longer than the others and is also quite common, if not universal. I think it has to do with how the world was made rather than the graphics rendering per se.