I think this is still the definition

The deep web,[1] invisible web,[2] or hidden web[3] are parts of the World Wide Web whose contents are not indexed by standard web search-engine programs.

But I don’t remember a time that 4chan didn’t appear on search results, it’s just that the content disappears so quickly that the result you do get (and you get some results) aren’t worth much, so they’re demphasized. But I remember a long while back searching on Google with “site:4chan.org” and getting results.