The issue is that subpixel antialiasing is removed, which affects both retina displays and, more heavily, the non-retina display.
In 10.12, Apple reimplemented its PDF renderer. With that, the subpixel antialiasing was removed. This affects all apps that use it (Preview, Safari, Quicklook, etc.).
In 10.14, Apple turned of subpixel antialiasing in the rest of the operating system (dialogs, buttons, etc.). However, this can be re-enabled by running:
defaults write -g CGFontRenderingFontSmoothingDisabled -bool NO
In 10.15 and 11.0, subpixel antialiasing is completely gone, and can't be re-enabled.
And there isn't a way to use the 10.11's PDF renderer in 10.12 onwards. However, at least a little bit of luck (until 10.13, maybe 10.14): with above setting you find subpixel antialiasing in some a third party PDF apps that were compiled using the 10.11 frameworks, like PDFpen 9 or PDF Expert 2.4.15.
