I’m trying to extend a sitemap.xml to include some custom data so I don’t have to store it in a separate database but just in-file. From what I found online, some people say xml namespaces should allow custom fields to work with sitemaps ( 1, 2 ), others say search engines might reject it ( 1, 2 ). Which one is true? Is there an objective truth, at least per engine?

And as for how, from what I understand, I need to define my namespace schema in an xsd file.
This is what I’ve come up with, but I don’t know how to even test it. Could someone tell me if this is correct?

sitemap.xml

<?xml version="1.0" encoding="UTF-8"?>
<urlset xmlns:saleor="http://example.org/xml-schemas/saleor-ref.xsd"
    xmlns="http://www.sitemaps.org/schemas/sitemap/0.9">
    <url>
        <loc>https://example.org/cat/product</loc>
        <lastmod>2020-02-17</lastmod>
        <saleor:ref>
            <saleor:id>coolsaleoridofaproduct</saleor:id>
            <saleor:category>coolidoftheparentcategory</saleor:category>
        </saleor:ref>
    </url>
    <url>
        <loc>http://example.org/cat</loc>
        <lastmod>2021-02-17</lastmod>
        <saleor:ref>
            <saleor:id>coolsaleoridofaproduct</saleor:id>
        </saleor:ref>
    </url>
</urlset>

saleor-ref.xsd – I assume has to be hosted at targetNamespaces url?

<?xml version="1.0"?>
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema" targetNamespace="http://example.org/xml-schemas/saleor-ref.xsd">
  <xs:element name="ref">
    <xs:complexType>
      <xs:sequence>
        <xs:element name="id" type="xs:string"/>
        <xs:element name="category-id" type="xs:string" minOccurs="0"/>
      </xs:sequence>
    </xs:complexType>
  </xs:element>
</xs:schema>

Thanks <3