The Pixel 2 camera software is entirely separate from the operating system and so it is updated separately. You download the camera app from the Google Play Store. I don't see how an operating system update could affect the camera
More noise and grain are, of course, the result of the phone's having to use a tiny 1/2.3 sensor. The physical sensor didn't change with the update so inherent hardware noise wouldn't change.
With enough light there is little to no noise with the Pixel 2 sensor. With low light there will, of course, be noise and grain. What could change, however, is how the camera software is dealing with noise.
Software mitigates noise and grain at the expense of sharpness. Noise and sharpness are a tradeoff. Google has opted to tilt the scale toward sharpness which would result in a bit more noise but result in a sharper image. At the smaller sizes we use on the Internet this is a good choice. With a wall sized enlargement however then maybe not so much.