Disable Add to Cart button based on flag

Hiding the Add to Cart button in Drupal Commerce 2.x is easy. Just alter the commerce_order_item_add_to_cart_form form, and set the disabled property of the button to true.

use Drupal\Core\Form\FormStateInterface;

function MY_MODULE_form_alter(&$form, FormStateInterface $form_state, $form_id) {
  if (strpos($form_id, 'commerce_order_item_add_to_cart_form') !== FALSE) {
    $product = $form_state->getFormObject()->getEntity()->getPurchasedEntity()->getProduct();
    
    $flag_service = \Drupal::service('flag');
    $flag = $flag_service->getFlagById('sold_out');
    $flagging = $flag_service->getFlagging($flag, $product);

    if ($flagging !== NULL ) {
      $form['actions']['submit']['#value'] = t('Out of stock');
      $form['actions']['submit']['#disabled'] = TRUE;
    }
  }
}