designtopack/public/site/plugins/comments/routes/delete.php

<?php

return [
  'pattern' => '(:all)delete-comment.json',
  'method' => 'POST',
  'action'  => function () {
      $json = file_get_contents('php://input');
      $data = json_decode($json);

      $page = page($data->location->page->uri);
      $project = page($data->location->project->uri);
      $file = $page->file($data->location->file->uuid);
      $isReply = $data->location->parentId ?? false;

      $comments = $file->comments()->isEmpty() == true ? [] : Yaml::decode($file->comments()->value());

      foreach ($comments as $key => &$comment) {
        if ($isReply) {
          if ($comment['id'] === $data->location->parentId) {
            foreach ($comment['replies'] as $replyKey => $reply) {
              if ($reply['id'] === $data->id) {
                unset($comment['replies'][$replyKey]);
                $comment['replies'] = array_values($comment['replies']);
              }
            }
          }
        } else {
          if ($comment['id'] === $data->id) {
            unset($comments[$key]);
          }
        }
      }

      $comments = array_values($comments);

      $newFile = $file->update([
        'comments' => $comments
      ]);
      
      echo json_encode(getFileData($newFile));
      
      kirby()->user()->deleteNotification($project, $data->id);

      exit;
  }
];