Not that I don't believe you, but that just hasn't been my experience with Google at all, and I use it extensively for research tracking down fairly obscure sources with presumably close-to-zero traffic by putting in quoted sentence fragments.

Do you have an example of a website that is linked to from other sites (so Google can find it) where a quoted search for a unique sentence fragment from it doesn't list it as a result?

I've just never heard authoritatively of Google removing old content from results, especially since it seems antithetical to the company's mission of organizing all the world's information. And seeing how so much of the value of search (and purchasing and content and so much else) is in the "long tail", it seems like it would be an extremely counterproductive strategy.