Can a private pilot land on an abandoned runway in southern California's remote areas?
There are a few unpaved runways near scenic hillsides, ideal for camping or swimming.
As noted in the comments it depends on who owns the land and what the local regulations are. With permission and proper operations it may be ok.
However if the runway was formerly active and is now no longer maintained its likely a bad idea to try and land on it. The FAA can even get you under their catch all 91.13 Careless or reckless operation.
There are procedures for closing a runway and marking it as such (you can find the AC here). Some times the "x" used to mark the closure is a physical structure that can make it dangerous to land on the runway. In other cases the runway may no longer be suitable for use or there may be active construction going on. While the runway may appear to be ok on the surface that does not mean it is safe to land.