Not everything. A lot. But stuff published this year still hasn't made its way there. And not all social sciences.

source: dissertating

(When I need something I can't get through my library, #icanhazpdf, find on scihub, or other googley means; I just email the author. Though that can take some time.)

In my experience, the only hole in shadow libraries is that they lack scans of old papers on non-trendy subjects. Interestingly, it seems the humanities don't have analogously useful sites.