2 Retired has it right.
All you knew in the past about reservations or no reservations has changed now due to COVID.
We are nearing the end of a 6-week trip, and we have found that almost everyplace is disallowing walk-up, no-reservation camping. We've been all up and down the east coast and all throughout Florida. If you don't have reservations, you generally can't camp. Not because of demand, but because they don't want you to spontaneously show up. The Florida state parks were the worst. Generally, in the past, you could get a walk-up site the day-of, but not now. Lots of sites are going unoccupied because of this. One host told us that if the state finds that the park is collecting revenue on a non-reservable site (in other words, if they sell it to you as a walk-in) they will be "in trouble."
This is what we've found concerning state parks. Other types of campgrounds may have different policies. Point is - COVID has changed everything.
-Speak