They appear to have change something in the past few minutes. When I first opened this HN thread it showed me Google's homepage. Now I'm also seeing that redirect.

NSA has just shut down the proxy. The link was a Google Doodles game.

You can just replace captcha.nsa.gov with www.google.com to see what it used to serve up: https://www.google.com/logos/2019/loteria/rc2/loteria19.html...