I currently have a 1.15.5 K8s cluster hosted in azure that I will soon look into upgrading. I can see this should still be possible:
$ az aks get-upgrades --resource-group my-cluster --name my-cluster --output table
Name ResourceGroup MasterVersion Upgrades
------- --------------- --------------- ----------------
default my-cluster 1.15.5 1.16.13, 1.16.15
But going forward where do I find some public documentation for the exact date a specific version of k8s will no longer be valid for an upgrade?
You can find the information you are looking for in Supported Kubernetes versions in Azure Kubernetes Service (AKS).
From the docs:
AKS publishes a pre-announcement with the planned date of a new version release and respective old version deprecation on the AKS Release notes at least 30 days prior to removal.
More information can be found here.
To find out which versions are currently available for your subscription and region, use the az aks get-versions command.
az aks get-versions --location eastus --output table
output: