A 410 HTTP header means the deleted content is gone forever.
When I remove a page from my website, I never put it back and it is gone forever.
I want Google to index nothing other than the existing pages in my website.
So what is the downside of using 410 instead of 404 HTTP header? It is not a common practice so I expect that there is some kind of disadvantage or harm in doing so.
Search engines (including Google) index neither "404 Not Found" URLs nor "410 Gone" URLs. From an indexing perspective using 410 as the default instead of 404 isn't going to be any different.
The biggest difference between the two statuses is how quickly Google removes a page from the index when it no longer exists:
Thus using 404 as the default error but 410 for pages that are intentionally removed gives you the greatest power. It offers protection for your site when you make accidental removals but allows you quickly remove pages from the search index by setting a 410 error for them when they are deleted.
A 410 error means "this page was here but is now gone" and a 404 error means "this page could not be found." 410 errors are a subset of 404 errors and the semantics of a 410 status would not be appropriate for a URL that has never had content on it.
I would not recommend using a 410 error as a default error instead of "404 Not Found." The semantics of 410 aren't right for the default, and using 404 as the default gives your website some SEO protection from accidental deletions.
