Not working hooks "SpecialListusersHeaderForm" and "SpecialListusersHeader" (tested on version 1.30 and 1.31). Changing the "out" variable does not add text to the Special:ListUsers page.
The problem occurs because of an error in the array names in the file UsersPager.php: https://doc.wikimedia.org/mediawiki-core/master/php/UsersPager_8php_source.html
In the lines 336 and 352 instead of the name of the array "$formDescriptor" used "$formDescript i or".