Now that the Taxonomy term reference field has been removed in favor of Entity reference it begs the question how one is supposed to specifically target taxonomy term reference fields in Twig templates and in your CSS.

The following (which currently involves checking for the existence of a particular method) may not be very pretty, but it works.

In your THEME.theme:

function d8webel_preprocess_field(&$variables, $hook) {
    if ($variables['field_type'] == 'entity_reference') {
        $items = $variables['element']['#items'];
        $fd = $items->getFieldDefinition();
        if (method_exists($fd,'getSettings')) {
            $settings = $fd->getSettings();
            if ($settings['target_type'] == 'taxonomy_term') {
                $variables['is_term'] = TRUE;
            }
        }
    }
}

Then in field.html.twig (assuming you have say copied it from Classy) you can then merge in a CSS class to flag taxonomy term fields like this:

{%
  set classes = [
    'field',
    'field--name-' ~ field_name|clean_class,
    'field--type-' ~ field_type|clean_class,
    'field--label-' ~ label_display,
  ]
%}

...

{% if is_term %}
    {%
    set classes = classes|merge(['is-term'])   
    %}
{% endif %}

...

Then in your CSS you can target taxonomy term items like this:

div.field.field--type-entity-reference.is-term div.field__item
{
...
}