remove email field from registration form From core\modules\user\src\AccountForm.php, user module developer added following comment to Email fields.

 // **The mail field is NOT required if account originally had no mail set**
    // and the user performing the edit has 'administer users' permission.
    // This allows users without email address to be edited and deleted.
    // Also see \Drupal\user\Plugin\Validation\Constraint\UserMailRequired.

    $form['account']['mail'] = array(
      '#type' =&gt; 'email',
      '#title' =&gt; $this-&gt;t('Email address'),
      '#description' =&gt; $this-&gt;t('A valid email address. All emails from the system will be sent to this address. The email address is not made public and will only be used if you wish to receive a new password or wish to receive certain news or notifications by email.'),
      '#required' =&gt; !(!$account-&gt;getEmail() &amp;&amp; $user-&gt;hasPermission('administer users')),
      '#default_value' =&gt; (!$register ? $account-&gt;getEmail() : ''),
    );


I do not understand what is "originally had no mail set" mean. 
it looks like , there is a config option which can turn off Email requirement, but I am unable to find the config option.
Mean that you can set an account without e-mail, as admin or someone that have rights, in url '/admin/people/create'. But when a visitor goes to 'user/register', he must specify a e-mail, because someday an e-mail will only be used if you wish to receive a new password, as description says.

There are two types of accounts, one with e-mail (like a visitor that created a new account), and another without e-mail (admin created a new user). This is why AccountForm check if user have an e-mail and check if user is admin or have 'users' permission, to set e-mail field required or not.

There is no reason to a visitor create a account without an e-mail, there is no other way to admin/system contact him for any kind of situation. In other hand if an admin create a new user, somehow he will able to contact the new user.