Just opening this for the record...
4 years ago, the SVG WG resolved to remove SVGTests.hasExtension due to lack of use and being a poor API.
It was removed from Chromium at that time and has been removed from Firefox recently
Sent to webkit-dev too: https://lists.webkit.org/pipermail/webkit-dev/2019-June/030659.html
As I mentioned in webkit-dev, the key question here is whether there is any web content that relies on this feature either intentionally or accidentally; e.g. iOS specific web content.
SVG 2 (W3C Recommendation): https://www.w3.org/TR/SVG2/types.html#InterfaceSVGTests
SVG 1.1 (W3C Recommendation): https://www.w3.org/TR/SVG11/types.html#InterfaceSVGTests
@Ryosuke: It looks like LayoutTests/web-platform-tests/mathml/relations/html5-tree/mathml/relations/html5-tree/required-extensions-1.html (test for SVGTests.hasExtension and MathML namespace) was removed in bug 200470. Should we import it back into LayoutTests/mathml?