Hm, I really don't know for sure. All extreme planet traits (radioactive, aquatic, frozen, barren, toxic, but also Precursor Shield) have a line "<IsExtreme>true</IsExtreme>" in the XML, but also some precursor planets in DLC4_PlanetTraitDefs.xml (Precursor Quarantine, Precursor Experimental World, Precursor Prison). So I *assume* the galaxy setting for extreme planets affects all those in the same way.