Search engines may not properly index pages that return unsuccessful HTTP status codes. Lighthouse considers any HTTP status code between 400 and 599 (inclusive) to be unsuccessful and flags them in the report:
How to ensure search engines can index your page
When a page is requested, ensure that your server returns a 2XX or 3XX HTTP status code. Search engines may not properly index pages with 4XX or 5XX status codes. Learn more in Fix unsuccessful HTTP status codes.
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