The Valve Software site's URL is not www.valve.com. It does not list Half-Life 2 as a product. Sierra's Half-Life page does not refer to Half-Life 2. (Jurie goes to the Sierra site and searches for Half-Life 2.) The Half-Life 2 page on the Sierra site contains a button that says 'Official Site'. This leads to www.gameoftheyear.com. This site contains another Offical Site button. This button leads to www.half-life.com, which appears to contain the information I was actually looking for.
All three Half-Life 2 sites allow me to sign up for the Half-Life 2 newsletter.