It uses opengl, it requires version 4.3 i think. Several versions higher than our current requirements to try cater to older hardware.But is it possible to consider replacing opengl with this in the future? or an option for people to choose? I mean maybe some ways to activate ReShade in a easier way, without messing around with replacement files.
As for bundling it with SM? No. They specify not to redistribute or upload their files to anywhere other than the reshade git.
Free to a user and being able to distribute are very different, It would breach copyright laws.
Plus there is the inefficiency of using expensive reshade shaders when we could create and optimize our own later on down the road specifically for SM.