If wgLegacyJavaScriptGlobals is set to false then wgCascadeableLevels is not in mw.config at all.
I'm doing a clean up of several Html::inlineScipt uses today, I'll assign this one to me as a reminder.
Bug can be reproduced in any version that has wgLegacyJavaScriptGlobals (added in r87856 - 1.18)
Version: 1.18.x
Severity: major