Issue #3111645 by bobbygryzynger, catch: Uninstall entity_reference module and...

<?php

/**

 * @file

 * Entity Reference install functions.

 */

/**

 * Implements hook_requirements().

 */

function entity_reference_requirements($phase) {

  $requirements = [];

  if ($phase === 'install') {

    $requirements['entity_reference'] = [

      'title' => t('Entity Reference: Deprecated'),

      'description' => t('Entity Reference is deprecated, all functionality has been moved to Core.'),

      'value' => \Drupal::VERSION,

      'severity' => REQUIREMENT_ERROR,

    ];

  }

  return $requirements;

}

<?php

/**

 * @file

 * Deprecated. All its functionality has been moved to Core. This empty module

 * will be removed in Drupal 9.0.x.

 */

<?php

namespace Drupal\entity_reference;

use Drupal\Core\Field\Plugin\Field\FieldType\EntityReferenceItem;

/**

 * Deprecated. Alternative implementation of the 'entity_reference' field type.

 *

 * @deprecated in drupal:8.0.0 and is removed from drupal:9.0.0. Use

 *   \Drupal\Core\Field\Plugin\Field\FieldType\EntityReferenceItem instead.

 *

 * @see \Drupal\Core\Field\Plugin\Field\FieldType\EntityReferenceItem

 */

class ConfigurableEntityReferenceItem extends EntityReferenceItem {}

<?php

namespace Drupal\entity_reference\Plugin\views\display;

use Drupal\views\Plugin\views\display\EntityReference as ViewsEntityReference;

/**

 * Deprecated. The plugin that handles an EntityReference display.

 *

 * @deprecated in drupal:8.0.0 and is removed from drupal:9.0.0. Use

 *   \Drupal\views\Plugin\views\display\EntityReference instead.

 *

 * @see \Drupal\views\Plugin\views\display\EntityReference

 */

class EntityReference extends ViewsEntityReference {}

<?php

namespace Drupal\entity_reference\Plugin\views\row;

use Drupal\views\Plugin\views\row\EntityReference as ViewsEntityReference;

/**

 * EntityReference row plugin.

 *

 * @deprecated in drupal:8.0.0 and is removed from drupal:9.0.0. Use

 *   \Drupal\views\Plugin\views\row\EntityReference instead.

 *

 * @see \Drupal\views\Plugin\views\row\EntityReference

 */

class EntityReference extends ViewsEntityReference {}