CC @nigeltao

I don't see why this has to be in the standard library. Floyd-Steinberg is one of many dithering algorithms (as your https://pkg.go.dev/github.com/makeworld-the-better-one/dither/v2 shows) and part of the image/gif API design (and the existing Options.Drawer field) is that you can bring your own dithering implementations (with a Strength field or whatever else you want).

On the other hand, anything in the standard library has to be supported forever and, once released, its API is frozen forever. There's a high bar for adding anything to the stdlib at this point.