$form_state['clicked_button'] removed in favor of $form_state['triggering_element']

$form_state['clicked_button'] has been removed in favor of $form_state['triggering_element']. This is to support AJAX form submissions, which are not always triggered by pressing a button. $form_state['clicked_button'] was deprecated in Drupal 7 already, so it is safe to switch to $form_state['triggering_element'] in both Drupal 7 and Drupal 8 alike. The usage is identical.

in a later time in Drupal 8, $form_state was converted to an object. You can now get the triggering element like this:

$trigger = $form_state->getTriggeringElement();