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

<?php

namespace Cpcommerce\ShippingNotifications\Model\Config\Source;

use Magento\Framework\Data\OptionSourceInterface;
use Magento\Catalog\Model\ResourceModel\Product\Attribute\CollectionFactory;

class Attributes implements OptionSourceInterface{
    
    public function __construct(
        CollectionFactory $collectionFactory
    ){
        $this->collectionFactory = $collectionFactory;
    }
    
    public function toOptionArray()
    {
        $options = [];
        foreach($this->getOptions() as $optionValue => $optionLabel){
            $options[] = ['value' => $optionValue, 'label' => $optionLabel];
        }

        return $options;
    }

    public function getOptions()
    {
        $collection = $this->collectionFactory->create();
        $collection->addFieldToFilter('main_table.frontend_input', 'select');
        $collection->addOrder('attribute_code', 'asc');

        $options = ['' => _('-- Empty --')];
        foreach($collection->getItems() as $attribute){
            $options[$attribute->getAttributeCode()] = $attribute->getAttributeCode();
        }

        return $options;
    }
}