You're assuming that YC has this information from all the startups they fund. I doubt this for two reasons: YC is probably too busy to spend time collecting spreadsheets from everybody; and from what I've seen, most startups aren't careful enough about their accounting to have the data in the first place.
I doubt there's a formal collection but I'd be equally surprised if they don't have early stage projections from many of the companies. As silly as they can be, they're often part of pitches.
From my limited personal experience, every startup I've chatted about this with has some sort of forecast though the degree of seriousness with which is was constructed and is used varies.
I'm now working at my 3rd YC startup. Sometimes we show our Leftronic graphs to investors, but other than I have never seen someone send hard data to YC. YC is very hands off. You do a weekly dinner the first few weeks, then you do a demo day, and then after that maybe sometimes you get an email once every few months asking how you're doing. Maybe you go visit when you have a question or are bored. That's about it. Other than that you just write code and talk to users.
