Here's a previous thread with the same question. Conveniently, it's also the same answer. 🙂
We haven't retired any members lately. Some member have had us close their accounts because they got a job with Apple (Apple prohibits participation by employees) or because they didn't like the forum rules (stricter than a lot of other forum sites).
Some long-time members have simply disappeared, and some we thought were gone have returned. It's the nature of an evolving community.
With the size of the membership these days, it's less likely for any one member to really be "well-known".