shipping-notifications/ShippingNotifications/Model/Config/Source/ShippingCategories.php

<?php

namespace Cpcommerce\ShippingNotifications\Model\Config\Source;

use Magento\Framework\Data\OptionSourceInterface;
use Magento\Catalog\Model\Product\Attribute\Repository;
use \Magento\Framework\App\Config\ScopeConfigInterface;

class ShippingCategories implements OptionSourceInterface
{
    private $attribute;
    private $scopeConfig;

    public function __construct(
        Repository $attribute,
        ScopeConfigInterface $scopeConfig
    ){
        $this->attribute = $attribute;
        $this->scopeConfig = $scopeConfig;
    }

    public function toOptionArray()
    {
        $options = [];

        $attributeCode = $this->scopeConfig->getValue('cpcommerce_shipping_notifications/general/attribute',\Magento\Store\Model\ScopeInterface::SCOPE_STORE);

        $shippingCategories = $this->attribute->get($attributeCode)->getOptions();

        foreach($shippingCategories AS $shippingCategory){
            $options[] = [
                'value' => $shippingCategory->getValue(),
                'label' => $shippingCategory->getLabel()
            ];
        }
        
        return $options;
    }
}