DisableAutoGroupAssignDefault.php

<?php
/**
 * Copyright © Magento, Inc. All rights reserved.
 * See COPYING.txt for license details.
 */
namespace Magento\Customer\Model\Config\Backend\CreateAccount;

class DisableAutoGroupAssignDefault extends \Magento\Framework\App\Config\Value
{
    /**
     * @var \Magento\Eav\Model\Config
     */
    protected $eavConfig;

    /**
     * @param \Magento\Framework\Model\Context $context
     * @param \Magento\Framework\Registry $registry
     * @param \Magento\Framework\App\Config\ScopeConfigInterface $config
     * @param \Magento\Framework\App\Cache\TypeListInterface $cacheTypeList
     * @param \Magento\Eav\Model\Config $eavConfig
     * @param \Magento\Framework\Model\ResourceModel\AbstractResource $resource
     * @param \Magento\Framework\Data\Collection\AbstractDb $resourceCollection
     * @param array $data
     */
    public function __construct(
        \Magento\Framework\Model\Context $context,
        \Magento\Framework\Registry $registry,
        \Magento\Framework\App\Config\ScopeConfigInterface $config,
        \Magento\Framework\App\Cache\TypeListInterface $cacheTypeList,
        \Magento\Eav\Model\Config $eavConfig,
        \Magento\Framework\Model\ResourceModel\AbstractResource $resource = null,
        \Magento\Framework\Data\Collection\AbstractDb $resourceCollection = null,
        array $data = []
    ) {
        $this->eavConfig = $eavConfig;
        parent::__construct($context, $registry, $config, $cacheTypeList, $resource, $resourceCollection, $data);
    }

    /**
     * Save EAV default value after save
     *
     * @return $this
     */
    public function afterSave()
    {
        $result = parent::afterSave();

        $attributeObject = $this->eavConfig->getAttribute('customer', 'disable_auto_group_change');
        $attributeObject->setData('default_value', $this->getValue());
        $attributeObject->save();

        return $result;
    }
}