Azure Kubernetes Service—Retiring support for Kubernetes versions 1.11 and 1.12

On Monday, December 9, 2019 (2019-12-09), we’re removing support for Kubernetes versions older than the latest version (N), and two minor versions prior (N-2), because support for Kubernetes 1.15 will become available on that date. Customers running Kubernetes versions older than Kubernetes 1.13.x must upgrade by Monday, December 9, 2019, to remain in support.
Source: Azure Roadmap