Back in 2007 I tried to get to Mt. Thayer using the obvious ridge that once had a road (according to Topo maps). I made it about 0.4 miles, with multiple sections requiring crawling on the ground--it was pretty miserable. A friend of mine tried the same thing a year ago with no luck. A few weeks ago, another friend said it was "open". Indeed, it's entirely navigatable.
This old trail is 100% legal Sierra Azul OSP goodness starting near El Sombroso. There is an old sign highlighting it is not a through trail. Much later there is a surprise OSP sign saying "private property ahead" (easy to miss). That property is part of Sierra Azul now. It's Sierra Azul all the way to this cache and a bit farther. The structures on Mt. Thayer were razed a while ago so it's a nice summit begging for visitation. . .Have a suspicion the whole route will be official at some future time, at which point this cache will defintely not be Hardcore!
I hiked up from the Kennedy entrance. 16 miles RT with 4k ft of elevation gain--a fine hike. That includes going a bit farther after the cache because I was Almost Thayer. There are plenty of ways to get to the starting point near El Sombroso--choose your favorite. Hopefully the trail will not be retaken by nature. The far end of the trail has multiple animal cameras, so it might be maintained now. There is, in fact, a camera in the same clearing as the cache. The camera has a tag promising that any pictures of humans will be deleted.
If you like hiking as much as caching (or more!), be sure to visit all the caches in the Hardcore Hiker series. Perhaps you'll show up on the list of those who have found caches in the series. Are you hardcore?