Safety Cover for Decommissioned (Empty) Swimming Pool
If you are really decommissioning it, a truckload of stone (or multiple truckloads of stone) will do nicely and comparatively inexpensively. Ain't nobody going to fall into it or drown in it.

If you have doubts about ever using it again (or some future homeowner doing so) then leaving water and freeze damage prevention (if a freezing climate) pillows in it with a cover over is a lot more easily reversible.