gohugo: How to exclude content from sitemap.xml

The problem

Some content is shown on the sitemap and there is no clear way how to exclude it using the gohugo framework. For example it's common to list the 404 page in the sitemap while using hugo.

Solution

A possible solution would be to provide your own sitemap.xml template and specify sitemap_exclude: true in the frontmatter:

<urlset xmlns="http://www.sitemaps.org/schemas/sitemap/0.9"
  xmlns:xhtml="http://www.w3.org/1999/xhtml">
  {{ range .Data.Pages }}{{ if ne .Params.sitemap_exclude true }}
  <url>
    <loc>{{ .Permalink }}</loc>{{ if not .Lastmod.IsZero }}
    <lastmod>{{ safeHTML ( .Lastmod.Format "2006-01-02T15:04:05-07:00" ) }}</lastmod>{{ end }}{{ with .Sitemap.ChangeFreq }}
    <changefreq>{{ . }}</changefreq>{{ end }}{{ if ge .Sitemap.Priority 0.0 }}
    <priority>{{ .Sitemap.Priority }}</priority>{{ end }}{{ if .IsTranslated }}{{ range .Translations }}
    <xhtml:link
                rel="alternate"
                hreflang="{{ .Lang }}"
                href="{{ .Permalink }}"
                />{{ end }}
    <xhtml:link
                rel="alternate"
                hreflang="{{ .Lang }}"
                href="{{ .Permalink }}"
                />{{ end }}
  </url>
  {{ end }}{{ end }}
</urlset>