You can now select if you would like your AKS nodes to be deleted or deallocated when scaled down.When a node is deallocated, the attached storage as well as the container image is still present on the node. When scaling up your deployments, all deallocated nodes will be started first before provisioning new nodes. This allows for faster operation speeds as your deployment leverages cached images. Scale-down mode allows you to no longer have to pre-provision nodes and pre-pull container images.
With the move of preview to GA, scale-down mode now supports spot node pools.
For more info, visit: https://docs.microsoft.com/en-us/azure/aks/scale-down-mode
Source: Azure Roadmap