Taxonomy term view mode suggestion

You need to use underscores in your suggestion (but continue to use dashes in your template file name):

function linkup_theme_suggestions_taxonomy_term_alter(&$suggestions, $vars, $hook) {
if($vars['elements']['#view_mode'] == 'teaser') {
$suggestions[] = 'taxonomy_term__teaser'; // note underscores
}
}
you could also leave out your condition and do:

function linkup_theme_suggestions_taxonomy_term_alter(&$suggestions, $vars, $hook) {
$suggestions[] = 'taxonomy_term__' . $vars['elements']['#view_mode'];
}