Restrict new user registration via hook_install() instead of config/install.

config/install/user.settings.yml

@@ -1,16 +0,0 @@
-anonymous: Anonymous
-verify_mail: true
-notify:
-  cancel_confirm: true
-  password_reset: true
-  status_activated: true
-  status_blocked: false
-  status_canceled: false
-  register_admin_created: true
-  register_no_approval_required: true
-  register_pending_approval: true
-register: admin_only
-cancel_method: user_cancel_block
-password_reset_timeout: 86400
-password_strength: true
-langcode: en

farm.install

@@ -14,6 +14,9 @@ function farm_install() {
 
   // Use private file system by default.
   \Drupal::configFactory()->getEditable('system.file')->set('default_scheme', 'private')->save();
+
+  // Only allow admins to register new users.
+  \Drupal::configFactory()->getEditable('user.settings')->set('register', 'admin_only')->save();
 }
 
 /**