Kite

Due to security/privacy concerns, JSON REST API (WP API) only allows post meta data (custom field value) for authenticated users. 
This plugin is an easy way to make post meta available when unauthenticated.
After activate the plugin, go to WP API Customizer option page. 
Click the plus icon to add rows. 
Set JSON attributes and custom field names in your posts that you want to use when unauthenticated. 
The JSON REST API will provide the data as “post_meta” like the following:
<pre><code>"post_meta":{"your_attribute_a":"Custom Field A value","your_attribute_b":"Custom Field B value"}
</code></pre>

WP API Customizer

Make post meta data (custom field values) available for JSON REST API (WP API) when unauthenticated.

Tran Ngoc Tuan Anh

eLightUp

Meta Box

thaoha

MB Yoast SEO Integration is a free extension for <a href="https://metabox.io" rel="nofollow ugc">Meta Box</a> plugin which allows developers to add content of custom fields to Yoast SEO Content Analysis to have better SEO score.
<blockquote>
Meta Box Lite 
 We recommend using <a href="https://metabox.io/lite/" rel="nofollow ugc">Meta Box Lite</a>, a feature-rich free UI version of Meta Box that provides UI and all free features for managing custom fields and dynamic content on WordPress, including post types, taxonomies, custom fields, and relationships.
</blockquote>
There are situations when we create custom fields to store real content for the posts. They are actually displayed in the frontend. Search engines such as Google or Bing see them and analyze them. But by default, Yoast SEO plugin takes only post content to analyze for SEO score and gives us feedback based on the post content. The plugin doesn’t analyze the all the content that appears to the search engine, which is a big missing part and that sometimes confuses us in SEO term.
So, the plugin MB Yoast SEO Integration fixes that problem by allowing us to add custom fields to the content analyzed by Yoast SEO plugin. This way, the Yoast SEO plugin and search engine will see the same content and we will have a correct advice for SEO content as well as correct SEO score.
<h3>Plugin Links</h3>
<ul>
<li><a href="https://metabox.io/plugins/meta-box-yoast-seo/" rel="nofollow ugc">Homepage</a></li>
<li><a href="https://github.com/rilwis/mb-yoast-seo" rel="nofollow ugc">Github repo</a></li>
<li><a href="https://metabox.io/plugins/" rel="nofollow ugc">View other premium extensions</a></li>
</ul>

MB Yoast SEO Integration

Add content of Meta Box custom fields to Yoast SEO Content Analysis.

WP API Customizer vs MB Yoast SEO Integration