Follow-up from #1984766: Change notice: Start relying on Request/Response objects for cache handling.
HTTP headers are now handed by the Symfony Request/Response objects. The legacy http header functions probably aren't doing anything. All uses of these functions need to be updated to use the objects. The functions themselves should be removed in a followup patch.
The affected functions include:
_drupal_set_preferred_header_name
drupal_add_http_header
drupal_get_http_header
drupal_page_header
drupal_send_headers