Valve was intending to reveal information about Dota 2 Patch 7.00 but as it announced the details, the site went offline likely due to the amount of traffic.
Since Patch 6.00 was released 11 years ago, the interest in Patch 7.00 has been massive. Dota 2 still commands millions of players and is big within the eSports arena. In fact, the announcement for Patch 7.00 came last night at the Boston Major where Team OG claimed victory over Ad Finem.
Currently, players attempting to view the website are greeted with a 503 Service Unavailable error.
Dota 2 is available now as a free-to-play game on Mac, PC and Steam OS via Steam.
If you want a sample of what you will be able to see when the site is live, you can view a semi-cached version over on the Internet Archive Wayback Machine. Although, be warned that there is a lot of placeholder text so it is mainly for viewing images and animated gifs that you would want to go there now.