taxonomy_admin_vocabulary_title_callback() has been removed and replaced by entity_page_label()

In Drupal 7, there is a custom vocabulary title callback, mostly used for admin pages in core: taxonomy_admin_vocabulary_title_callback(). In Drupal 8, this function has been removed. Instead, use the entity_page_label/$entity->label() functions of the entity system.

Drupal 7:

return taxonomy_admin_vocabulary_title_callback($vocabulary);

Drupal 8:

return entity_page_label($vocabulary);
// Alternative solution:
return $vocabulary->label();