disable_field_autocomplete/disable_field_autocomplete.module

<?php

/**
 * @file
 * Contains disable_field_autocomplete.module.
 */

use Drupal\Core\Routing\RouteMatchInterface;
use Drupal\taxonomy\Entity\Term;

/**
 * Implements hook_help().
 */
function disable_field_autocomplete_help($route_name, RouteMatchInterface $route_match) {
  switch ($route_name) {
    case 'help.page.facet_field_formatter':
      $output = '';
      $output .= '<h3>' . t('About') . '</h3>';
      $output .= '<p>' . t('Disable autocomplete for TypedRelation fields') . '</p>';
      return $output;

    default:
  }
}

/**
 * Implements hook_form_alter().
 */
function disable_field_autocomplete_form_node_islandora_object_edit_form_alter(&$form, \Drupal\Core\Form\FormStateInterface $form_state, $form_id) {
  $form['#validate'][] = 'disable_field_autocomplete_node_edit_validate';
}


/**
 * Extra Islandora Ojbect node validation required due to lacking autocomplete.
 * @param $form
 * @param \Drupal\Core\Form\FormStateInterface $form_state
 * @return void
 */
function disable_field_autocomplete_node_edit_validate(&$form, \Drupal\Core\Form\FormStateInterface $form_state){
  $contributors = &$form_state->getValue('field_linked_agent');
  $contributor_count = 0;
  foreach ($contributors as &$contributor) {
    if (is_array($contributor) && isset($contributor['target_id']) && is_array($contributor['target_id'])) {
      $entity = ($contributor['target_id']['entity']);
      $name = $entity->getName();
      if (isset($name)) {
        try {
          $term_id = disable_field_autocomplete_get_term_id($name);
          if (isset($term_id) && $term_id > 0) {
            $contributor['target_id'] = $term_id;
          }
        } catch (Exception $e) {
          $form_state->setError($form['field_linked_agent']['widget'][$contributor_count],
            t('The entered Term %name is not unique across Vocabularies.  You will need to include
      the term id in brackets.  For Example "%name (119)", where 119 is the term id.', array('%name' => $name)));
        }
      }
    }
    $contributor_count++;
  }
  return;
}

/**
 * Checks for existing terms.
 *   Since autocommplete is off we need to check ourselves.
 * @param $term_name
 * @return int
 * @throws \Drupal\Component\Plugin\Exception\InvalidPluginDefinitionException
 * @throws \Drupal\Component\Plugin\Exception\PluginNotFoundException
 */
function disable_field_autocomplete_get_term_id($term_name) {
  $storage = \Drupal::entityTypeManager()->getStorage('taxonomy_term');
  $terms = $storage->loadByProperties([
    'name' => $term_name,
    'vid' => ['Person', 'Contributors', 'Host Contributors', 'Series Contributors'],
  ]);

  if ($terms) {
    if (count ($terms) > 1) {
      throw new Exception ('The entered Term is not unique across Vocabularies.  You will need to include
      the term id in brackets.  For Example "term value (119)", where 119 is the term id.');
    }
    $term = reset($terms);
  }
  if(!isset($term)) {
    return -1;
  }
  $tid = $term->id();
  return $tid;
}