Feature summary
In a standard MediaWiki installation configured for a single language, Special:WithoutInterwiki is redundant because there are no interwiki links that would need listing.
Nevertheless, it is listed in Special:SpecialPages and adds to the clutter there. It also has a performance impact (likely very small, but nonzero) because something is checking and producing this list in the background.
Propose that this special page be enabled by a config flag such as $wgEnableWithoutInterwikiSpecialPage that is set to FALSE by default, but which can be set to TRUE in localsettings.php where needed (e.g. WMF wikis).