Jeff Starr

This plugin does one thing: disables the WP REST API for visitors who are not logged into WordPress. No configuration required.
This plugin works with only 22 short lines of code (less than 2KB). So it is super lightweight, fast, and effective.
Note: This plugin completely disables the WordPress REST API. So it is not recommended if you need the WP REST API for any functionality.
<h3>Features</h3>
<ul>
<li>Disable REST/JSON for visitors (not logged in)</li>
<li>Disables REST header in HTTP response for all users</li>
<li>Disables REST links in HTML head for all users</li>
<li>100% plug-and-play, set-it-and-forget solution</li>
</ul>
The fast, simple way to prevent abuse of your site’s REST/JSON API
How does it work? That depends on which version of WordPress you are using..
WordPress v4.7 and beyond
For WordPress 4.7 and better, this plugin completely disables the WP REST API unless the user is logged into WordPress.
<ul>
<li>For logged-in users, WP REST API works normally</li>
<li>For logged-out users, WP REST API is disabled</li>
</ul>
What happens if logged-out visitor makes a JSON/REST request? They will get only a simple message:
“rest_login_required: REST API restricted to authenticated users.”
This message may customized via the filter hook, <code>disable_wp_rest_api_error</code>. Check out <a href="https://wordpress.org/support/topic/not-entirely-for-non-techies/#post-12014965" rel="ugc">this post</a> for an example of how to do it.
Older versions of WordPress
For WordPress versions less than 4.7, this plugin simply disables all REST API functionality for all users.
More information available below in the FAQs section.
<h3>Privacy</h3>
This plugin does not collect or store any user data. It does not set any cookies, and it does not connect to any third-party locations. Thus, this plugin does not affect user privacy in any way. If anything it improves user privacy, as it protects potentially sensitive information from being displayed/accessed via REST API.
Disable WP REST API is developed and maintained by <a href="https://x.com/perishable" rel="nofollow ugc">Jeff Starr</a>, 15-year <a href="https://plugin-planet.com/" rel="nofollow ugc">WordPress developer</a> and <a href="https://books.perishablepress.com/" rel="nofollow ugc">book author</a>.
<h3>Support development of this plugin</h3>
I develop and maintain this free plugin with love for the WordPress community. To show support, you can <a href="https://monzillamedia.com/donate.html" rel="nofollow ugc">make a donation</a> or purchase one of my books:
<ul>
<li><a href="https://wp-tao.com/" rel="nofollow ugc">The Tao of WordPress</a></li>
<li><a href="https://digwp.com/" rel="nofollow ugc">Digging into WordPress</a></li>
<li><a href="https://htaccessbook.com/" rel="nofollow ugc">.htaccess made easy</a></li>
<li><a href="https://wp-tao.com/wordpress-themes-book/" rel="nofollow ugc">WordPress Themes In Depth</a></li>
<li><a href="https://books.perishablepress.com/downloads/wizards-collection-sql-recipes-wordpress/" rel="nofollow ugc">Wizard’s SQL Recipes for WordPress</a></li>
</ul>
And/or purchase one of my premium WordPress plugins:
<ul>
<li><a href="https://plugin-planet.com/bbq-pro/" rel="nofollow ugc">BBQ Pro</a> – Blazing fast WordPress firewall</li>
<li><a href="https://plugin-planet.com/blackhole-pro/" rel="nofollow ugc">Blackhole Pro</a> – Automatically block bad bots</li>
<li><a href="https://plugin-planet.com/banhammer-pro/" rel="nofollow ugc">Banhammer Pro</a> – Monitor traffic and ban the bad guys</li>
<li><a href="https://plugin-planet.com/ga-google-analytics-pro/" rel="nofollow ugc">GA Google Analytics Pro</a> – Connect WordPress to Google Analytics</li>
<li><a href="https://plugin-planet.com/head-meta-pro/" rel="nofollow ugc">Head Meta Pro</a> – Ultimate Meta Tags for WordPress</li>
<li><a href="https://plugin-planet.com/simple-ajax-chat-pro/" rel="nofollow ugc">Simple Ajax Chat Pro</a> – Unlimited chat rooms</li>
<li><a href="https://plugin-planet.com/usp-pro/" rel="nofollow ugc">USP Pro</a> – Unlimited front-end forms</li>
</ul>
Links, tweets and likes also appreciated. Thank you! 🙂

Disable WP REST API

while the plugin si deadsimple and very low resource impact, keep in mind that there are plugins that need the rest to work with no auth (ex. contact form 7).

dead simple and efficient, but be aware

Disables the WP REST API for visitors not logged into WordPress.

svrooij

Since wordpress version 4.9.? wordpress includes the functionality provided by this plugin in the core of wordpress.
Checkout https://developer.wordpress.org/rest-api/using-the-rest-api/global-parameters/#_fields for functionality included in the wordpress core, 
they made it even better by filtering the amount of data pulled from the database.
This plugin will no longer be maintained since it has been superseded by the core functionality.
The <a href="https://wordpress.org/plugins/rest-api/" rel="ugc">wp-rest-api-v2</a> returns a lot of properties. 
It could be very useful (or mobile-data-friendly) to only return the properties needed by the application.
If you only want titles and links of some articles it doesn’t make sense to return the content or the excerpt.
This isn’t limited to posts, it also works on custom posttypes, categories, pages, terms, taxonomies and comments.
Instead of returning:
<pre><code>{
 "id": 2138,
 "date": "2015-10-25T15:31:03",
 "guid": {
 "rendered": "http://worldofict.nl/?p=2138"
 },
 "modified": "2015-10-25T15:31:03",
 "modified_gmt": "2015-10-25T14:31:03",
 "slug": "rechtenvrije-fotos",
 "type": "post",
 "link": "http://worldofict.nl/tip/2138-rechtenvrije-fotos/",
 "title": {
 "rendered": "Rechtenvrije foto’s"
 },
 "content": {
 "rendered": ".. A lot of content .. "
 },
 "excerpt": {
 "rendered": " .. A lot of content ..."
 },
 "author": 2,
 "featured_image": 2139,
 "comment_status": "open",
 "ping_status": "open",
 "sticky": false,
 "format": "standard",
 //.. even more tags ....
}
</code></pre>
It can return (with <code>fields=id,title.rendered,link</code> as GET parameter)
<pre><code>{
 "id": 2138,
 "link": "http://worldofict.nl/tip/2138-rechtenvrije-fotos/",
 "title": {
 "rendered": "Rechtenvrije foto’s"
 }
}
</code></pre>
<h4>Notes</h4>
<ol>
<li>If you specify fields so it wouldn’t return data the default response is send back to the client.</li>
<li>(for developers) something wrong with this plugin? <a href="https://github.com/svrooij/rest-api-filter-fields/" rel="nofollow ugc">Github</a></li>
<li>If you liked the plugin <a href="https://github.com/sponsors/svrooij" rel="nofollow ugc">consider sponsoring me</a></li>
</ol>

REST API – Filter Fields

This is hands-down the best REST API extension available.

Best REST API plugin

Filter the properties returned by the Wordpress rest api V2 (only needed until wordpress 4.9)

Disable WP REST API vs REST API – Filter Fields