Get the delta on the host entity for a paragraph.

GetHostDelta.php

<?php

use Drupal\paragraphs\ParagraphInterface;

/**
 * Get the delta on the host entity for a paragraph.
 */
class GetHostDelta {

  public function __invoke(ParagraphInterface $paragraph): ?string {
    $host = $paragraph->getParentEntity();
    $host_field_name = !$host ? '' : $paragraph->get('parent_field_name')->value;
    $items = !$host_field_name ? [] : $host->get($host_field_name);
    foreach ($items as $item) {
      if ($item->get('target_id')->getValue() == $paragraph->id()) {
        return (string) $item->getName();
      }
    }

    return NULL;
  }
}