This function is no longer required. Email fields in Backdrop are using the HTML5 #email element now, so there is no need for a special validator for user forms.

If an email needs to be validated outside the Form API, it is advised to use valid_email_address() instead.

Introduced in branch: 
1.0.x
Introduced in version: 
1.0.0
Impacts: 
Module developers
Related Github Issues: 
https://github.com/backdrop/backdrop-issues/issues/108
https://github.com/backdrop/backdrop-issues/issues/1416