At DefaultSettings.php#L3709, there is
if ( window.wgRestrictionEdit ) { ... }
and
if ( mw.config.exists('wgRestrictionEdit') ) { ... }
However this variable is an array, not a boolean (see T133289), so it should be checking the length of the array instead:
if ( mw.config.exists('wgRestrictionEdit').length ) { ... }
This will always be undefined in this case. Please see the exact code of the example:
if ( mw.config.exists('wgRestrictionEdit') ) { ... }
it does not use mw.config.get(...) which will return an array for wgRestrictionEdit, however, it uses mw.config.exists, which:
Check if one or more keys exist.
and, per doc, will always return a boolean:
exists( selection ) : boolean
(Source: https://doc.wikimedia.org/mediawiki-core/master/js/#!/api/mw.Map)
So checking for a boolean here is correct, as we're not checking that there're any entries in the array, but instead checking, if the config variable even exists (~ is defined).
This looks like to be invalid then?