This book was reviewed in the Wall Street Journal yesterday:
https://www.amazon.com/Trail-Lost-Relentless-Missing-Pacific/dp/0306831953
It mentions the David O'Sullivan case, and others. I bumped into two SAR people who were searching for O’Sullivan. This was three or four months after he disappeared. We were on the Willow Creek Trail, not the PCT. When I mentioned that, they said that area had been well-searched, and it was time to explore other possibilities.
Evidently still not found:
https://www.sandiegouniontribune.com/columnists/story/2022-04-16/five-years-since-hiker-david-osullivan-disappeared-in-southern-california