This interface should probably have been removed as part of the pathSegList ( issue 539385 ), since it's not very useful without it. (It produces as output an index into the that list of SVGPathSegs.)
It was probably kept around because it (mistakenly) wasn't part of the SVGPathSegDOM counter... I wouldn't expect usage to be high.
The "SVG Paths" draft has a similar method, but with a different name and a slightly different signature: https://svgwg.org/specs/paths/#InterfaceSVGPathElement
Comment 1 by foolip@chromium.org
, Feb 16 2017