Independent and unrelated to Observatory project, but if anyone is curious to dig deeper into Alexa top ~500K and how it changed over time, take a look at the HTTP Archive dataset. Some examples @ https://discuss.httparchive.org/latest.

E.g, to identify sites with CSP: https://discuss.httparchive.org/t/which-sites-have-content-s....

You can access raw HAR captures, as well as pre-aggregated stats via BigQuery.