$Site.AdminEmailAddress to Be Deprecated in Summer ’19

To better secure your Sites, we are deprecating the $Site.AdminEmailAddress merge field. After the field is deprecated, it will return an empty string instead of the Site Contact value for the current site. You can still access the admin’s email address using SOQL queries.

Where: This change applies to Sites, accessed through Lightning Experience and Salesforce Classic in Enterprise, Performance, Unlimited, and Developer editions.

How: To access the admin’s email address, you can use these SOQL queries within an Apex controller.
SELECT Admin.Email
FROM Site
WHERE Id=\'' + Site.getSiteId() + '\'
SELECT Email
FROM User
WHERE Id=\'' + Site.getAdminId() + '\'