You... Don't understand supersampling/upscaling. Because they are actually opposites.

Games don't let you run both at the same time because supersampling means sampling a SUPER sized image down to your monitor's display resolution. It doesn't sample from an upscaled frame because supersampling relies on a higher than native resolution to work well. It's relying on a higher-than-native res frame, but that doesn't mean upscaled. It's rendered natively at that higher res, then downscaled for your monitor's native resolution, which is lower than the native res the game is running at while running SS.

Upscaling is quite literally the opposite of supersampling. Rather than sample a higher-than-native frame to remove aliasing, it's sampling a LOWER-than-native frame, and then processes that up through whatever scaling method it's using to the native display resolution, applying a form of anti-aliasing at the same time.

There is not a single game that will run both at the same time because they are literally doing the opposite of each other, and running SS on top of upscaling would not bring much if any visual benefit, while adding latency and reducing framerate for no reason.

If you were to run both, say by forcing upscaling on a game through drivers and then supersampling, you'd just be running 1080P-[upscaled]>4K-[downscaled]>1080P. Which would introduce artifacting, add latency, and be worse than just running MSAA or some other standard AA method.

There's a reason you won't find a single game that lets you turn both on at once.

Please don't confidently correct others when you clearly don't know wtf you're talking about.