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.

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This plugin will no longer be maintained since it has been superseded by the core functionality.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)

RESTless vs REST API – Filter Fields