"It depends". The core APIs of Kubernetes like pods and services work pretty much the same everywhere, or at least if you are getting into provider specific behavior you would know it since the provider name would be in the annotation. But each vendor does have their own extensions. For example, GKE offers integration with GCP IAM permissions as an alternative to Kuberenetes' internal RBAC system. If you use that, then switching is that much harder. The more provider-specific annotations and extensions you use, the more work it will be to switch.