Check below step If you want to display out of stock products at last on product listing page in Magento 2.

You need to develop a custom extension (Here, I’m assuming that you know how to create custom extension in Magento 2).

We need to use MagentoCatalogSearchModelResourceModelFulltextCollection for product collection.

1. Create app/code/Magemonkeys/OutOfStockAtLast/etc/di.xml and add below code.

<?xml version="1.0"?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:ObjectManager/etc/config.xsd">
    <type name="MagentoCatalogSearchModelResourceModelFulltextCollection">
        <plugin name="Magemonkeys_OutOfStockAtLast::OutofstockLast" type="MagemonkeysOutOfStockAtLastPluginProductProductListToolbar"/>
    </type>
</config>

2. Create app/code/Magemonkeys/OutOfStockAtLast/Plugin/Product/ProductList/Toolbar.php and add below code.

<?php
namespace MagemonkeysOutOfStockAtLastPluginProductProductList;
class Toolbar
{
    
    public function beforeLoad(
        MagentoCatalogSearchModelResourceModelFulltextCollection $subject, 
        $printQuery = false, 
        $logQuery = false
    )
    {
        $order = $subject->getSelect()->getPart(Zend_Db_Select::ORDER);
        $oosorder = array('is_salable DESC');
        $subject->getSelect()->reset(Zend_Db_Select::ORDER);
        $subject->getSelect()->order($oosorder);
        return [$printQuery, $logQuery];
    }
}

There you go. I hope following above steps will let you display out of stock product at last on catalog page in Magento 2.

Happy coding!