There's not really such a thing as a concrete "date of death" in many cases, and very rarely will there be a public one -- I'm familiar with a few names on this list that never actually made a formal public announcement and kept the website running for months to well over a year after everyone was laid off

The methodology of the aggregator might have been as simple as "ping every YC company's listed website, check the response" with some light hand curation, which suggests that the number presented is just a lower bound on the number of dead YC startups