Not trying to be snarky - but the way it's worded in the post makes it seem like it's the norm moving forward but also included for all YC companies:

"But starting in the Summer of 2014, we added a pro rata provision to our standard investment documents, and starting now, we're going to aim to support all YC companies in future financing rounds by doing our pro rata. We will try to do this for every company in every round with a post-money valuation of $250 million or less."