The problem with subpixel antialiasing (the rgba antialiasing), is that it is impossible to do properly with hardware acceleration when you need transparent surfaces or any rendering with an alpha channel in general. So you need software fallback for those cases, which slows down the rendering pipe eg in web browsers. for 4K screens is easier to just drop subpixel antialiasing and just rely on good old antialiasing instead, since it's no longer needed. I'm thinking most of the complaints with the newer rendering stacks are people still using FHD screen resolutions.