Some agents are opening up bookings for Disney World 2021, but do not do it yet.
TUI, Virgin and other travel agents have started to sell 2021 holidays. I know we all are crazy excited and want to get holiday's booked as soon as possible, but if you're planning to stay at a Walt Disney World resort do not book now.
You might have heard about free dining which is the best deal Disney do. The free dining offer tends to run from April for the following year. So for 2020 stays, the free dining offer began on April 2019. So don't book your 2021 Disney World hotel holiday until the free dining offer opens around April of 2020. Besides, Disney don't officially release room and ticket prices until then anyway. Keep in mind you cannot book now and add free dining later. Mark your calendar for April 2020!
However, if you plan to stay at a hotel off-site (for example, International drive) then you're safe to book early if you want to.