A namespace has been stuck in Terminating for hours. What is the most likely cause?

Answer: Leftover finalizers, or an aggregated API that is unavailable

Explanation

The namespace controller deletes every object in the namespace and waits for them to disappear. An object with a finalizer stays until its controller removes the finalizer, so if that controller was uninstalled, deletion hangs. An unavailable APIService blocks it too, because the controller can't list those resources. 'kubectl get namespace <name> -o yaml' shows the reason in status.conditions.