A soft 404 error is the term given to a URL that cannot be found on a web server, with the server returning a 200-level code rather than a 404 error or a 301 redirect to another URL. A 200-level code denotes success, a 404 code denotes “not found”, and a 301 code denotes that the server is to redirect the web user onto a new URL.
Soft 404s are considered an issue by search engines as the 200-level success code can mean that webpages that cannot load are included in search results. For search engines, this provides a poor service to web users.