Currently, it is not possible to obtain a list of existing font-feature values. You can only add or reset all (AddFontFeature/ResetFontFeatures). Neither the Pascal API nor skia2d provide the appropriate methods.

In the source class (skparagraph/include/TextStyle.h), this is handled using
size_t getFontFeatureNumber() const { return fFontFeatures.size(); }
std::vector getFontFeatures() const { return fFontFeatures; }

ps A similar situation with Shadows.