Exclude nodes from search API based on field value

NodeExcludeProcessor.php

<?php

namespace Drupal\my_module\Plugin\search_api\processor;

use Drupal\search_api\Processor\ProcessorPluginBase;

/**
 * Excludes entities marked as 'excluded' from being indexes.
 *
 * @SearchApiProcessor(
 *   id = "my_module_exclude_processor",
 *   label = @Translation("My module exclude processor"),
 *   description = @Translation("Exclude nodes from being indexed."),
 *   stages = {
 *     "alter_items" = -50
 *   },
 *   locked = true,
 * )
 */
class NodeExcludeProcessor extends ProcessorPluginBase {

  /**
   * {@inheritdoc}
   */
  public function alterIndexedItems(array &$items) {
    /** @var \Drupal\search_api\Item\ItemInterface $item */
    foreach ($items as $item_id => $item) {
      $object = $item->getOriginalObject()->getValue();

      if ($object->getEntityTypeId() === 'node' && $object->getType() === 'my_node_type') {
        if ($object->field_my_field->value === 'a value') {
          unset($items[$item_id]);
          continue;
        }
      }
    }
  }

}