The history file of mediawiki/core contains the following sentence:
* BREAKING CHANGE: Skins built for MediaWiki 1.15 and earlier that do not use the "headelement" template key are no longer supported. Setting $useHeadElement = false; is no longer supported and will not cause old keys like "headlinks", "skinnameclass", etc. to be defined.
But many skins setting this property, which is unused and should be removed.