To a search engine, a URL is just a string. Slashes are just characters in that string. How many slashes there are does not make a difference to the search engine spidering your site*. What can matter is your link structure - if you hold to strictly to the modular design, where the deep pages in your structure are only linked to by the index for example, the search engines may take that as a signal that the pages are less important to the overall site - that may give you the same result (less crawls and lower rankings) just for a different reason. There is also a design principle that users should not have to dig too deeply into a site structure to find the content they are looking for (3 click rule, IIRC).

*Note: There is a theoretical limit to how long that string should be. Generally, try to keep it as short as possible, with relevant keywords in the string. This will help not only giving the search engines clues to what the page is about, but also may help improve the click through rate from the SERPs as users skim the results.

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The best way to learn anything, is to question everything.