Why does Drupal (8) not discover my custom taxonomy term template?
You need to use underscores in your suggestion (but continue to use dashes in your template file name):

/**
 * Implements hook_theme_suggestions_HOOK_alter() for taxonomies.
 *
 * {@inheritdoc}
 */
function HOOK_theme_suggestions_taxonomy_term_alter(&amp;$suggestions, $vars, $hook) {
  if($vars['elements']['#view_mode'] == 'teaser') {
    $suggestions[] = 'taxonomy_term__teaser'; // note underscores
  }
}


you could also leave out your condition and do:

/**
 * Implements hook_theme_suggestions_HOOK_alter() for taxonomies.
 *
 * {@inheritdoc}
 */
function HOOK_theme_suggestions_taxonomy_term_alter(&amp;$suggestions, $vars, $hook) {      
  $suggestions[] = 'taxonomy_term__' . $vars['elements']['#view_mode'];
}