The problem is everyone has their own definition of successful. Is it breaking even? 20 employees? Being profitable? A $1million exit? A $1billion exit? Everyone rejected by YC wants to be that rare case.
It's a two way street and sometimes the company may be happy they didn't get into YC because it opened different doors that led to success. Maybe in some cases YC could have closed some doors that were crucial.
Many companies don't even apply to YC because it's geared towards tech companies that do not require capital (and when they do eventually, it's for people, not physical resources).
I wonder if YC already tracks this, maybe informally? It would be a good means of testing their own selection processes. Then again, they probably focus more on the "false positives" (YC startups that did poorly) than "false negatives" (the ones that got away).
Obviously they don't want to accept just anyone, that doesn't work for various signal, scale, attention and effectiveness reasons. They do however accept (positive) outliers without traditional credentials, which is the opposite of what a traditional company does. A traditional company is looking for safety, VCs and YC are looking for maximum leverage (not the typical financial two-fat-tails kind, the only-upsides kind that Nassim Taleb et al. likes).
As for your question, I can't imagine a reality where they don't track that sort of thing.
I'm always happy to see people succeed, especially people I fired at one point. The beautiful thing about our field is that it isn't a zero sum game.
EDIT: none of the successful people I know spend time on regret