North Campground will remain First-Come, First-Served and open year-round.
Rec.gov is in the process of adding reservations for Sunset Campground to recreation.gov
but no update on when online reservations will be available. The official NPS Camping in Bryce Canyon
page states that reservation window for Sunset Campground will be from May 20 - October 15, 2020. Reservations, when made available, will be on a 6-month rolling basis.
For more info on Sunset Campground reservations you might try calling the Rec.gov Reservation line (877) 444-6777.