The release notes for 2019 sp02, have a small bullet point regarding de-noiser improvements:
"Integrated an updated version of the Denoiser leading to significant improvements in handling very noisy images"
I wanted to shed a bit more light on this point, as I think the improvements are quite significant. As you can see in this image, there is a dramatic reduction in "blotchy artifacts" typically seen in lower pass counts, in this case, just 10 passes...ie so the non-denoised image was VERY noisy. Also, the general coloration at earlier stages of denoising more closely matches ground truth final colors.