The theme.inc file has a @deprecated instruction. This has been added by the issue #2369987: Remove SafeMarkup::set() from 'head' title on template_preprocess_html but the parent issue change record doesn't document this precise point https://www.drupal.org/node/2296163
$variables['head_title'] = $head_title;
// @deprecated in Drupal 8.0.0, will be removed before Drupal 9.0.0.
$variables['head_title_array'] = $head_title;There isn't any example to how properly document a variable issue.