The CustomUserSignup extension (https://www.mediawiki.org/wiki/Extension:CustomUserSignup) is currently installed on the English Wikipedia and the test Wikipedia (cf. http://noc.wikimedia.org/conf/InitialiseSettings.php.txt):
'wmgCustomUserSignup' => array(
'default' => false,
'enwiki' => true,
'testwiki' => true,
I believe this extension is no longer necessary given that its associated project is now inactive and marked historical (https://outreach.wikimedia.org/wiki/Account_Creation_Improvement_Project).
As the extension is causing problems (e.g., bug 37786), it'd be nice if it could simply be uninstalled. I guess someone will need to check with the outreach folks?