UPDATE 9-2-20 Has your Sims 3 game been affected by the recent pixilation issues?
UPDATE: 3/8/2021 Cororon has been affected now and is determined to figure this out, yaaay! Please go to her thread on Nvidia and support her efforts to have Nvidia take a look at what's going on. ...
I think it's important to not confuse the game's normal color banding with the OP's issue. There's always been some amount of color banding in the game, mainly on smooth textures in areas with poor light. I used to go to great lengths to try to photoshop it out of screenshots when I'd make comics, so I've stared frustratedly at a lot of these ugly gradients over the years.
That said, there are definitely some examples in this thread where the color banding is way worse than usual, to the point of looking like old 256-color graphics. ZeeGee and Emily4331 have the best examples on the first two pages of the thread (especially Emily's where it's happening in broad daylight). But some of the other examples that have been posted here don't look unusual for this game at all and probably shouldn't be any cause for concern for those Simmers.
Image spam for reference, on different Windows versions and graphics cards:
2010 / Windows XP / NVIDIA GeForce 9600 GT https://i.imgur.com/c4Jd7k2.jpg
2017 / Windows 10 / NVIDIA GeForce 950 https://i.imgur.com/vMbVTjG.jpg
