The Kubernetes implementation of the Container Storage Interface (CSI) has been promoted to GA in the Kubernetes v1.13 release. Support for CSI was introduced as alpha in Kubernetes v1.9 release, and promoted to beta in the Kubernetes v1.10 release. The GA milestone indicates that Kubernetes users may depend on the feature and its API without fear of backwards incompatible changes in future causing regressions.
GA features are protected by the Kubernetes deprecation policy. Although prior to CSI Kubernetes provided a powerful volume plugin system, it was challenging to add support for new volume plugins to Kubernetes: volume plugins were “in-tree” meaning their code was part of the core Kubernetes code and shipped with the core Kubernetes binaries—vendors wanting to add support for their storage system to Kubernetes (or even fix a bug in an existing volume plugin) were forced to align with the Kubernetes release process. In addition, third-party storage code caused reliability and security issues in core Kubernetes binaries and the code was often difficult (and in some cases impossible) for Kubernetes maintainers to test and maintain.
Source: kubernetes.io