how do you not replace / by - when changing the Url key of products in magento 2

I needed to the exactly the same. I have done it by creating an around plugin to the formatUrlKey() method in \Magento\Catalog\Model\Product\Url

[EDIT] The best approach is to enable just the characters you want, so we still remove special characters and spaces. I am adding the code to allow only / and _

Just create a file: app/code/YourVendorName/YourModule/Plugin/FormatUrlKeyPlugin.php

<?php

declare(strict_types=1);

namespace YourVendorName\YourModule\Plugin;

/**
 * Class FormatUrlKeyPlugin
 * @package YourVendorName\YourModule\Plugin
 */
class FormatUrlKeyPlugin
{

    /**
     * FormatUrlKeyPlugin constructor.
     * @param Translit $translit
     */
    public function __construct(\Magento\Framework\Filter\Translit $translit)
    {
        $this->translit = $translit;
    }

    /**
     * Prevent Magento from replacing slashes in url_key
     *
     * @param \Magento\Catalog\Model\Product\Url $subject
     * @param callable $proceed
     * @param $str
     * @return mixed
     */
    public function aroundFormatUrlKey(\Magento\Catalog\Model\Product\Url $subject, callable $proceed, $str)
    {
        $str = preg_replace('#[^0-9a-z_/]+#i', '-', $this->translit->filter($str));
        $str = strtolower($str);
        $str = trim($str, '-');
        return $str;
    }
}

And add the plugin to the di.xml (create a new one if you don't have it at your module) app/code/YourVendorName/YourModule/etc/di.xml

<?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="Magento\Catalog\Model\Product\Url">
        <plugin name="FormatUrlKeyPlugin" type="YourVendorName\YourModule\Plugin\FormatUrlKeyPlugin" sortOrder="1" disabled="false"/>
    </type>
</config>