See T200247. In 1.32 (with https://gerrit.wikimedia.org/r/c/mediawiki/core/+/447647), these static member variables will be removed completely, causing anything that accesses them to break. BlueSpiceFoundation/Core.class.php directly accesses MagicWord::$mDoubleUnderscoreIDs and needs to be updated to use the GetDoubleUnderscoreIDs hook. I tried to fix it in https://gerrit.wikimedia.org/r/c/mediawiki/extensions/BlueSpiceFoundation/+/449165, but failed.
This change worked fine for me, so i restored it and marked the method as deprecated. I also added the hook handler base class, the BlueSpiceExtensions* (only 3 or 4) could use instead of this method.