Hello csoucy, I would honestly fill it if it is physically unstable you can have a pool contractor come out and have the measure the walls and any cracks found in the pool and he will be able to tell you if it is safe for you to leave it or not. Plenty of people fill their pools in here in AZ and that's not a problem its the case if you want to sell the property with a pool that need work or a giant garden haha But defiantly have a pull contractor or a inspector come by and look at it to give you more info.