My config.toml

config.toml

[outputFormats]
  [outputFormats.articles]
    baseName = "articles"
    mediaType = "application/xml"
    isPlainText = false
    protocol = "xml://"
    path = "/"
  
  [outputFormats.authors]
    baseName = "authors"
    mediaType = "application/xml"
    isPlainText = false
    protocol = "xml://"
    path = "/"
  
  [outputFormats.tags]
    baseName = "tags"
    mediaType = "application/xml"
    isPlainText = false
    protocol = "xml://"
    path = "/"
  
  [outputFormats.sitemapindex]
    baseName = "sitemapindex"
    mediaType = "application/xml"
    isPlainText = false
    protocol = "xml://"
    path = "/"
  
 [outputs]
  home = ["HTML", "articles", "authors", "tags", "sitemapindex"]

home.sitemapindex.xml

{{ printf "<?xml version=\"1.0\" encoding=\"UTF-8\"?>" | safeHTML }}
<sitemapindex xmlns="http://www.sitemaps.org/schemas/sitemap/0.9" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://www.sitemaps.org/schemas/sitemap/0.9 http://www.sitemaps.org/schemas/sitemap/0.9/siteindex.xsd">
  {{- range . }}
  <sitemap>
    <loc>{{ .SitemapAbsURL }}</loc>
    <lastmod>{{ .LastChange.Format "2006-01-02T15:04:05Z" | safeHTML }}</lastmod>
  </sitemap>
  {{- end }}
</sitemapindex>