Is there a limit on taxonomy hierarchy depth by default?
I just posted an answer to a similar question at https://drupal.stackexchange.com/a/199247/6488 but if you are wanting it to be a sitewide default you would:


remove the if ($form['#bundle'] == 'MY_VOCAB') from
hook_form_taxonomy_form_term_alter() 
Change the check against current_path() in the hook_preprocess_page()


Some clever person could turn this into a module!