Notify users a month after signup if they still don't have a confirmed email, warning them that they will not be able to reset their password.
Jared suggested this at https://www.mediawiki.org/wiki/Talk:Password_reset#Action_Items .
T42040: Special:PasswordReset could use some design love
T58074: Remind users who have entered an email address, but haven't confirmed it
T171200: Add a configuration option to make email mandatory during account creation
T215633: Emailability: Prompt users during email skip attempt