BjornW

RESTless is a tiny WordPress plugin which disables access to the REST API (available in WordPress since version 4.4) for non-authenticated users. 
This prevents usage of the REST API by the general public and limits access to those with login credentials. No REST for the wicked.
It also supports WordPress Multisite installations.
<h3>Notes</h3>
Credits:
Sword icon used in the WordPress plugin repository and found as /assets/icon* 
From the series <a href="http://www.toicon.com/series/sketchy" rel="nofollow ugc">‘Sketchy’</a> By <a href="http://www.toicon.com/authors/1" rel="nofollow ugc">Shannon E Thomas</a>
Thanks Shannon E Thomas and to[icon] for sharing your work!
WordPress repository banner image found in /assets/banner*:
Orginal painting: <a href="http://hdl.handle.net/10934/RM0001.COLLECT.319230" rel="nofollow ugc">‘Draak’</a> 
Drawn by: <a href="https://en.wikipedia.org/wiki/Utagawa_Kuniyoshi" rel="nofollow ugc">Utagawa Kuniyoshi</a> 
Dated around 1808 – 1861 
Repository: Rijksmuseum.nl 
Modified by: <a href="https://burobjorn.nl" rel="nofollow ugc">Björn Wijers</a>
Thanks Rijksmuseum!

RESTless

RESTless disables REST calls for non-authenticated requests.

Jeff Starr

Does one thing: Completely disables the WordPress REST API for visitors who are not logged into WordPress. No configuration required.
Important: This plugin completely disables the WP REST API for visitors who are NOT logged in to WordPress. So not recommended if your site needs the WP REST API for any non-logged users.
👉 The fast, simple way to prevent abuse of your site’s REST/JSON API 
👉 Protects your site’s REST data from all non-logged users and bots 
👉 Uses only 4KB of code, so super lightweight, fast, and effective
🛠️ Pro version available! <a href="https://plugin-planet.com/rest-pro-tools/" rel="nofollow ugc">Check out REST Pro Tools »</a>
<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>
How does it work?
This plugin completely disables the WP REST API unless the user is logged into WordPress.
<ul>
<li>For logged-in (authenticated) users, WP REST API works normally</li>
<li>For logged-out (unauthenticated) 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:
<pre><code>rest_login_required: REST API restricted to authenticated users.
</code></pre>
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.
<h3>Pro Version</h3>
🛠️ Check out the Pro version, <a href="https://plugin-planet.com/rest-pro-tools/" rel="nofollow ugc">REST Pro Tools</a>, loaded with many awesome features:
<ul>
<li>One-click disable all routes</li>
<li>One-click disable all /users routes</li>
<li>Disable any specific user routes based on role</li>
<li>Whitelist any user IDs</li>
<li>Whitelist any IP addresses</li>
<li>Customize the REST error message</li>
<li>Customize the REST response code</li>
<li>Always require or force SSL/TLS</li>
<li>Disable all JSONP shenanigans</li>
<li>One-click disable any REST API headers</li>
<li>Add any post meta (custom field) to REST API</li>
<li>Add any user meta (custom field) to REST API</li>
<li>Add routes for site profile and author profile</li>
<li>Add routes for featured images and post categories</li>
<li>Add routes for post taxonomies and terms</li>
<li>At-a-glance check status of REST API</li>
</ul>
The free version does only one thing: disables REST API for unauthenticated users. The PRO version can do that and much more! Take full control of the REST API with <a href="https://plugin-planet.com/rest-pro-tools/" rel="nofollow ugc">REST Pro Tools »</a>
<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/rest-pro-tools/" rel="nofollow ugc">REST Pro Tools</a> – Awesome tools for managing the WP REST API</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.

RESTless vs Disable WP REST API