You may have seen errors in your Google Search Console about soft 404’s and wondered what they are. According to Google, a soft 404 is when the crawler discovers a page that Google believes to be an error page, but returns a 200 OK response header.
This is a very important point since not returning an actual 404 response header on an error page negatively impacts your crawl budget and it means that Google needs to spend time crawling pages that you do not want crawled.
Google illustrates this with an example that clarifies that regardless of what a page says, the response header is most important.
In addition to returning a 404 code in response to a request for a page that doesn’t exist, the server will also display a 404 page. This may be a standard “File Not Found” message, or it could be a custom page designed to provide the user with additional information. The content of the page is entirely unrelated to the HTTP response returned by the server. Just because a page displays a 404 File Not Found message doesn’t mean that it’s a 404 page. It’s like a giraffe wearing a name tag that says “dog.” Just because it says it’s a dog, doesn’t mean it’s actually a dog. Similarly, just because a page says 404, doesn’t mean it’s returning a 404.
Going back to the initial question: Are soft 404’s bad? No, they won’t cause your site to be penalized, but you are causing Google to crawl your site in an inefficient manner. The best practice is to ensure that all error pages give a 404 or 410 response header.