I just took a look at the simulator files and it seems that they really forgot to add the bigger icons for iPad Pro. Even developers couldn't use them for their apps, as they only added support for it in the last Xcode update which was released on December 8. So there's hope, that they don't forget about this. Photo Booth for example is not even optimized for iPad Pro and is just a scaled up 9.7-inch app. Just like the Apple Store app and Garageband, as well.
Share Sheet icons are still 152 x 152, that's why they are not pixelated.