FieldInstanceDefaults.php

<?php

namespace Drupal\field\Plugin\migrate\process\d7;

use Drupal\migrate\MigrateExecutableInterface;
use Drupal\migrate\ProcessPluginBase;
use Drupal\migrate\Row;

/**
 * @MigrateProcessPlugin(
 *   id = "d7_field_instance_defaults"
 * )
 */
class FieldInstanceDefaults extends ProcessPluginBase {
    
    /**
     * {@inheritdoc}
     */
    public function transform($value, MigrateExecutableInterface $migrate_executable, Row $row, $destination_property) {
        list($default_value, $widget_settings) = $value;
        $widget_type = $widget_settings['type'];
        $default_value = $default_value ?: [];
        // In Drupal 7, the default value for email fields is stored in the key
        // 'email' while in Drupal 8 it is stored in the key 'value'.
        if ($widget_type == 'email_textfield' && $default_value) {
            $default_value[0]['value'] = $default_value[0]['email'];
            unset($default_value[0]['email']);
        }
        return $default_value;
    }

}