Setting a front matter variable for sitemap

support
1

If I set sitemap: false in frontmatter, how do I exclude those posts from showing e.g in sitemap?

2

1) Create a new file: layouts/_default/sitemap.xml

2) Copy the embedded template code to the file you just created.

3) Insert a conditional test within the range statement. It’s probably easier to opt-out than to opt-in.

For example... 
{{ printf "<?xml version=\"1.0\" encoding=\"utf-8\" standalone=\"yes\"?>" | safeHTML }}
<urlset xmlns="http://www.sitemaps.org/schemas/sitemap/0.9"
  xmlns:xhtml="http://www.w3.org/1999/xhtml">
  {{ range .Data.Pages }}
    {{- if not .Params.exclude_from_sitemap -}}
      {{- if .Permalink -}}
  <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="{{ .Language.Lang }}"
                href="{{ .Permalink }}"
                />{{ end }}
    <xhtml:link
                rel="alternate"
                hreflang="{{ .Language.Lang }}"
                href="{{ .Permalink }}"
                />{{ end }}
  </url>
      {{- end -}}
    {{- end -}}
  {{ end }}
</urlset>

There’s a very old issue and an associated pull request that would make this easier… not sure why it has not been implemented.