<!-- 
 Site Indexing toggled by key value of `private` in Page/Post Settings 
 404.html and 404.php are noindex, nofollow by default
 -->
 <meta name="robots" content=
 {{- if or (.Params.private) (in (string .Permalink) "404.html") (in (string .Permalink) "404.php") -}}
     "noindex, nofollow"
 {{- else -}}
     "index, follow"
 {{ end }} />

 <!-- Meta Keywords/Tags for Users to search for in Search Engines -->
 <meta name="keywords" content="
 {{- with .Params.tags }}
     {{- range $key, $item := . -}}
         {{- if ne $key 0 -}}
         ,
         {{- end -}}
         {{ with $.Site.GetPage (printf "/%s/%s" "tags" . ) }}
             {{- .Title -}}
         {{- end }}
     {{- end }}
 {{- else -}}
     Default, Keywords, That, Describe, Your, Page
 {{- end }}" />   

 <!-- Canonical URLs -->
 <link rel="canonical" href="{{ .Permalink }}" />
 <link rel="alternate" href="{{ .Permalink }}" hreflang="{{ .Site.LanguageCode }}" />

 <!-- Links to RSS Feed -->
 {{ range .AlternativeOutputFormats -}}
     {{ printf `<link rel="%s" type="%s" href="%s" title="%s" />` .Rel .MediaType.Type .Permalink $.Site.Title | safeHTML }}
 {{ end -}}

Add the default key/value private = true to /archetypes/default.md

 ---
 title: "{{ replace .Name "-" " " | title }}"
 date: {{ .Date }}
 draft: true

 ## Add this key with value 'false' for indexing by search engines enabled by default
 private: false
 ---

Usage

Add these keys and values to any page's settings:

+++
...
tags = ["fruit", "apple", "health"] # Fitting keywords that describe your post/page
private = true # true if you want it private/unlisted
+++

Preview

Public

Example Post

<meta name="keywords" content="Tag1, Tag2, Tag3" />
<meta name="robots" content="index, follow" />
<link rel="canonical" href="http://localhost:1313/" />
<link rel="alternate" href="http://localhost:1313/" hreflang="en-us" />
<link rel="alternate" type="application/rss+xml" href="http://localhost:1313/index.xml" title="Example Blog" />

Index

<meta name="keywords" content="default-tag1, default-tag2, default-tag3" />
<meta name="robots" content="index, follow" />
<link rel="canonical" href="http://localhost:1313/posts/example-post" />
<link rel="alternate" href="http://localhost:1313/posts/example-post" hreflang="en-us" />
<link rel="alternate" type="application/rss+xml" href="http://localhost:1313/index.xml" title="Example Blog" />

Private

404 Page is private by default

<meta name="keywords" content="Tag1, Tag2, Tag3" />
<meta name="robots" content="noindex, nofollow" />
<link rel="canonical" href="http://localhost:1313/404.html" />
<link rel="alternate" href="http://localhost:1313/404.html" hreflang="en-us" />