What is a soft 404?

A soft 404 means that a URL on your site returns a page telling the user that the page does not exist and also a 200-level (success) code to the browser. (In some cases, instead of a "not found" page, it might be a page with little or no usable content--for example, a sparsely populated or empty page.)

Why does it matter?

Returning a success code, rather than 404/410 (not found) or 301 (moved), is a bad practice. A success code tells search engines that there’s a real page at that URL. As a result, the page may be listed in search results, and search engines will continue trying to crawl that non-existent URL instead of spending time crawling your real pages.

What should I do?

• If your page is no longer available, and has no clear replacement, it should return a 404 (not found) or 410 (Gone) response code. Either code clearly tells both browsers and search engines that the page doesn’t exist. You can also display a custom 404 page to the user, if appropriate: for example, a page containing list of your most popular pages, or a link to your home page.
• If your page has moved or has a clear replacement, return a 301 (permanent redirect) to redirect the user as appropriate.

Use Fetch as Google (or other tools available on the web) to verify whether your URL is actually returning the correct code.

* Nguồn: Google Search Console