Update: No-Risk Mileage Award Space for American, Delta & United
American, Delta, and United all now have an award waiver in place, meaning it costs nothing to redeposit miles if you make a booking now and change your mind later. Only last month, fees were imposed to do this (albeit low fees). So, depending on your dates (see below), the risk of booking has been lowered.
American Airlines: waived award reinstatement and change fees for awards booked by May 31, 2020, for travel through September 30, 2020.