Some Cal state parks stop requiring reservations on October 1st and is first come first serve according to the American Land & Leisure campground manager who told me this in May. I just returned from short fishing camping trip at Lake Davis in Plumas National Forest and their Lightning tree campground is fully open with no reservations or fees. You must pack out your own trash and no running water and no hookups but some vault toilets are open. Only 5 campers in the 50 campground site as of yesterday. The other two campgrounds are closed for the season.
Must be flexible and camping during week is always successful for me. I don't like weekend noisy crowds and their non stop barking dogs.
I frequently boondock on northern Nevada vast BlM lands that are completely free and open.