ZF-11061: ext/mcrypt is only required by Zend_Service_ReCaptcha_MailHide
The manual's requirements page lists ext/mcrypt as a hard dependency of Zend_Service_Recaptcha.
There's no mention or usage of mcrypt in the "normal" Zend_Service_Recaptcha code. It appears that mcrypt is only required if you use the Zend_Service_ReCaptcha_MailHide sub-component.
So, it seems like the requirements page should either list mcrypt as a "Soft" dependency, or as a hard dependency on the MailHide sub-component.