only view selected taxonomy terms

BooleanTaxonomyTermFormatter.php

<?php
namespace Drupal\libevents\Plugin\Field\FieldFormatter;

use Drupal\Core\Field\FieldItemListInterface;
use Drupal\Core\Field\Plugin\Field\FieldFormatter\EntityReferenceEntityFormatter;
use Drupal\Core\Field\EntityReferenceFieldItemListInterface;

/**
 * Plugin implementation of the 'boolean_taxonomy_term_formatter' formatter.
 *
 * @FieldFormatter(
 *   id = "boolean_taxonomy_term_formatter",
 *   label = @Translation("Boolean Taxonomy Term Formatter"),
 *   field_types = {
 *     "entity_reference"
 *   }
 * )
 */
class BooleanTaxonomyTermFormatter extends EntityReferenceEntityFormatter {

  /**
   * {@inheritdoc}
   */
  protected function getEntitiesToView(EntityReferenceFieldItemListInterface $items, $langcode) {
    $entities = parent::getEntitiesToView($items, $langcode);

    $entity = $items->getEntity();
    $boolean_field_name = $this->getSetting('boolean_field_name');
    $taxonomy_term_field_name = $this->getSetting('taxonomy_term_field_name');

    $boolean_values = $entity->get($boolean_field_name)->getValue();
    $taxonomy_terms = $entity->get($taxonomy_term_field_name)->referencedEntities();

    $selected_term = null;

    if (!empty($boolean_values[0]['value'])) {
      $selected_term = $taxonomy_terms[0] ?? null;
    } elseif (!empty($boolean_values[1]['value'])) {
      $selected_term = $taxonomy_terms[1] ?? null;
    } else {
      $selected_term = $taxonomy_terms[2] ?? ($taxonomy_terms[0] ?? null);
    }

    return $selected_term ? [$selected_term] : [];
  }

  /**
   * {@inheritdoc}
   */
  public static function defaultSettings() {
    return [
      'boolean_field_name' => '',
      'taxonomy_term_field_name' => '',
      'view_mode' => 'default',
    ] + parent::defaultSettings();
  }

  /**
   * {@inheritdoc}
   */
  public function settingsForm(array $form, FormStateInterface $form_state) {
    $element = parent::settingsForm($form, $form_state);

    $element['boolean_field_name'] = [
      '#type' => 'textfield',
      '#title' => $this->t('Boolean field name'),
      '#default_value' => $this->getSetting('boolean_field_name'),
      '#required' => TRUE,
    ];
    $element['taxonomy_term_field_name'] = [
      '#type' => 'textfield',
      '#title' => $this->t('Taxonomy term field name'),
      '#default_value' => $this->getSetting('taxonomy_term_field_name'),
      '#required' => TRUE,
    ];
    $element['view_mode'] = [
      '#type' => 'textfield',
      '#title' => $this->t('View mode'),
      '#default_value' => $this->getSetting('view_mode'),
      '#required' => TRUE,
    ];

    return $element;
  }

  /**
   * {@inheritdoc}
   */
  public function settingsSummary() {
    $summary = [];

    $summary[] = $this->t('Boolean field: @boolean_field', ['@boolean_field' => $this->getSetting('boolean_field_name')]);
    $summary[] = $this->t('Taxonomy term field: @taxonomy_term_field', ['@taxonomy_term_field' => $this->getSetting('taxonomy_term_field_name')]);
    $summary[] = $this->t('View mode: @view_mode', ['@view_mode' => $this->getSetting('view_mode')]);

    return $summary;
  }
}