Fabio Moro

4zero4 makes your 404 pages useful. When a visitor reaches a URL that no longer 
exists, the plugin builds a search query from the requested address and redirects 
the visitor to your site’s search results, so they land on relevant content 
instead of an empty error page.
For example, a request for <code>/my-old-product-name/</code> that no longer resolves is 
turned into a search for “my old product name”.
Key points:
<ul>
<li>Zero configuration. Activate it and it works.</li>
<li>Temporary redirect (302). The original URL may become valid again, so the 
redirect is never cached as permanently moved.</li>
<li>Front-end only. It never interferes with the admin area, feeds or the REST 
API, and only acts on normal GET page views.</li>
<li>No external services. The plugin does not send any data anywhere and does 
not use tracking or analytics.</li>
</ul>
<h4>Developer hooks</h4>
The behaviour can be customised with filters:
<ul>
<li><code>p4zero4_search_terms</code> — change (or clear, to skip) the terms extracted from the URL.</li>
<li><code>p4zero4_redirect_url</code> — change (or clear, to cancel) the destination URL.</li>
<li><code>p4zero4_redirect_status</code> — change the HTTP status code (default <code>302</code>).</li>
</ul>

4zero4

Turn "page not found" (404) errors into a helpful search instead of a dead end.

Kaspars

Set any page to be used as the 404 error page under “Settings > Reading”.

Custom 404 Error Page

Use any static page as the 404 error page.

4zero4 vs Custom 404 Error Page