> Slug doesn't seem to support any of these, it is just "for a given point, am I in or am I out?", but it doesn't tell you by how much

It does give you an anti-aliased value between 0 and 1 that estimates how much of a pixel is being covered.

But this is a linear estimate based on horizontal and vertical distance to the Bezier curve. It does not look correct at long distances, which is why you shouldn't use it for outlines, drop shadows or the other cool things you can do with (M)SDF. A single pixel outline works fine but is not really legible with modern display resolutions (very thin lines).

For finding the minimum distance between a quadratic Bezier curve and a point would require solving a 3rd degree polynomial, where Slug's algorithm gets away with solving a quadratic equation per pixel. This is makes a big performance difference.