To be able to quantitatively compare Google-of-old with Google-of-today on the web content today, we'd have to have access to both. And we'd have to assume that Google-of-old's algorithms have not been figured-out and abused by the content that's there today. And certainly the content has changed: I wonder if you can look into archive.org by-date and run a search engine on it for comparison (I'd like to test if proportion of commercial content has grown up).
Without access to both Googles, the best you can do is compare across different search engines: special-cased ones like search.marginalia.nu can net you a quantitative feel for what exists out there that's less likely to be content marketing, but I am not sure if you can figure out where those pages rank in Google search results for the same terms programmatically?
You can also prepare for the future: record some data today, and compare in 10 years time.
Without access to both Googles, the best you can do is compare across different search engines: special-cased ones like search.marginalia.nu can net you a quantitative feel for what exists out there that's less likely to be content marketing, but I am not sure if you can figure out where those pages rank in Google search results for the same terms programmatically?
You can also prepare for the future: record some data today, and compare in 10 years time.