How do I print the help text description above the field for all textareas?
The description_display property is still working if supported by your active theme and I understand it is. Adding this to your mytheme.theme should do the trick :

function MYTHEME_preprocess_form_element(&amp;$variables) {
  if ($variables['element']['#type'] == 'textarea') {
    $variables['description_display'] = 'before';
  }
}


Note that this might not work with formatted fields from the field API as the description field is in a parent template, see text-format-wrapper.html.twig. It would be a little more complicated to have this working for those fields.