https://www.mediawiki.org/wiki/Extension:MobileFrontend/MobileMenu has many broken links and seems to be generally out of date
https://doc.wikimedia.org/mediawiki-core/master/php/MobileMenuHook_8php.html
https://github.com/wikimedia/mediawiki-skins-MinervaNeue/blob/master/includes/menu/Group.php
\MobileFrontend\MenuBuilder used in the example doesn't seem to exist anymore..
And the hook isn't in MobileFrontened anymore, it's in MinervaNeue, passing a Group (MediaWiki\Minerva\Menu\Group)
Hooks::run( 'MobileMenu', [ 'pageactions.overflow', &$group ] );