Forgive me, I can see that it does now. It seems like the code for the email physically doesn't exist in the new section.
I will log this as a bug, however for the time being, we should be able to use the same code as the old file. If you can revert back to the 1.8.10 version of the users module, and then apply these changes to the new awaiting activation section...
In ./admin/modules/user/awaiting_activation.php, line 72:
$query = $db->simple_select("users", "uid, usergroup, coppauser", "uid IN ({$user_ids})");
change this to:
$query = $db->simple_select("users", "uid, username, email, usergroup, coppauser", "uid IN ({$user_ids})");
Then, line 93:
$db->update_query("users", $updated_user, "uid='{$user['uid']}'");
After this, add:
$message = $lang->sprintf($lang->email_adminactivateaccount, $user['username'], $mybb->settings['bbname'], $mybb->settings['bburl']);
my_mail($user['email'], $lang->sprintf($lang->emailsubject_activateaccount, $mybb->settings['bbname']), $message);
Lastly, in ./inc/languages/english/admin/user_awaiting_activation.lang.php, at the end of the file, add this:
$l['emailsubject_activateaccount'] = "Account Activation at {1}";
$l['email_adminactivateaccount'] = "{1},
The administrator has activated your forum account on {2}.
To proceed, please go to
{3}
You will be able to login with the credentials you registered with.
Thank you,
{2} Staff";
This is just moving the existing language strings into the awaiting activation section language file.
This should then restore the original functionality.