It used to be believed by some SEO’s if you went too deep with your directory subfolder structure (over 4 subfolders deep – website.com/subfolder-1/subfolder-2/subfolder-3/subfolder-4/webpage.html) those webpages wouldn’t even be spidered at all by search engines: not true BTW!
The reason for this SEO myth is an artefact of common internal navigation linking structures, not subfolder structure per se.
The subfolder structure tends to mimic the link or navigation structure of a website and if it takes at least 4 links (clicks/taps) to go from the home page to the deepest webpages of a large website, some links might be missed by the search engine spiders and so some deep content might never be spidered at all: basically Googlebot never finds them.
Those deep linked webpages will tend to have one or a very small number of unimportant internal backlinks and so as links are very important to Google rankings they won’t be seen as important by Google and so might not be indexed: Google spiders most webpages it finds, but decides for various reasons they shouldn’t be included in the Google web search index.
Put another way if you only send one unimportant internal link to a webpage on your website and Googlebot rarely follows it, why would Google believe it’s an important webpage and rank it highly or even keep it in the Google web search index?
Reality is you can put your webpages as deep as you like subfolder wise (within reason) as long as they are linked correctly: try to go for no more than two taps/clicks from the home page, three at the very most for large websites.
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