User blog comment:DaNASCAT/Technical Update: March 20, 2017/@comment-35301431-20170321010743/@comment-2200849-20170321102341

AFAIK that namespace was used by the Maps extension (the vanilla Maps, not Wikia Maps). It is related to custom layer functionality but it's broken in the installed version 3.5 and was removed in version 3.7 entirely.