I don’t know about the data, but what I do know, is most established fund VCs believe this, and because of this, they feel they need to meet the companies well before demo day, or else everything is “picked over”.
Many great YC companies are funded before demo days, especially in prior years. Many others have plenty of interest, but still wait until after to pick the best partners from the suitors, especially if they are only selling say 10%. Others are funded by sources of capital that don’t even come to demo days. And still a key fourth group doesn’t really take off until 6-12 months after demo days.
That much I know.