ylsine

Do you have a blog that really only want ready by humans? Do think that 
<a href="http://google.com/" rel="nofollow ugc">robots</a> should never be able to read your blog, even if they ignore your robots.txt? Is privacy your number one concern? Do your friends think of you as “that total privacy nut…you know too much so.”? Then this plugin is for you. The 404er will make every public page of your blog return a <a href="http://en.wikipedia.org/wiki/HTTP_404" rel="nofollow ugc">404</a> instead of the normal <a href="http://en.wikipedia.org/wiki/List_of_HTTP_status_codes#2xx_Success" rel="nofollow ugc">200</a>. Yes, you could use the normal <a href="https://codex.wordpress.org/Settings_Privacy_SubPanel" rel="nofollow ugc">privacy subpanel</a>, but that would be sooo pedestrian. Step up to the real (excessive, <a href="http://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html#sec10.4.5" rel="nofollow ugc">RFC abbusing</a>) 404!
Note: If you did not detect any humor in the above text or you do not know what a 404 is, then, in all caps, DO NOT INSTALL THIS PLUGIN!

The 404er

This plugin will send a 404 header for every public page on your blog.

Dimitar Ivanov

HTTP Headers gives your control over the http headers returned by your blog or website.
Headers supported by HTTP Headers includes:
<ul>
<li>Access-Control-Allow-Origin</li>
<li>Access-Control-Allow-Credentials</li>
<li>Access-Control-Max-Age</li>
<li>Access-Control-Allow-Methods</li>
<li>Access-Control-Allow-Headers</li>
<li>Access-Control-Expose-Headers</li>
<li>Age </li>
<li>Content-Security-Policy</li>
<li>Content-Security-Policy-Report-Only</li>
<li>Cache-Control</li>
<li>Clear-Site-Data</li>
<li>Connection</li>
<li>Content-Encoding</li>
<li>Content-Type</li>
<li>Cross-Origin-Embedder-Policy</li>
<li>Cross-Origin-Opener-Policy</li>
<li>Cross-Origin-Resource-Policy</li>
<li>Expect-CT</li>
<li>Expires</li>
<li>Feature-Policy</li>
<li>NEL</li>
<li>Permissions-Policy</li>
<li>Pragma</li>
<li>P3P</li>
<li>Referrer-Policy</li>
<li>Report-To</li>
<li>Strict-Transport-Security</li>
<li>Timing-Allow-Origin</li>
<li>Vary</li>
<li>WWW-Authenticate</li>
<li>X-Content-Type-Options</li>
<li>X-DNS-Prefetch-Control</li>
<li>X-Download-Options</li>
<li>X-Frame-Options</li>
<li>X-Permitted-Cross-Domain-Policies</li>
<li>X-Powered-By</li>
<li>X-Robots-Tag</li>
<li>X-UA-Compatible</li>
<li>X-XSS-Protection</li>
</ul>

HTTP Headers

Great plugin with a huge amount of settings. Despite the complexity of the setup, the developers have added comments with links to full help. It helps a lot that the comments are also translated into Russian.
After setting up the plugin, be sure to check both the functionality of the site and the speed indicators. Some settings may slow down the site loading speed.

Wonderful

HTTP Headers adds CORS & security HTTP headers to your website.

The 404er vs HTTP Headers