If you want to uninstall istio on your Kubernetes cluster,
Delete istio-system namespace is not enough to uninstall istio.
Need to delete CRDs too.
Can be completely uninstall follow this command. If you installed using istioctl and default profile.
istioctl x uninstall --purge
kubectl delete ns istio-system
or
istioctl manifest generate --set profile=default | kubectl delete -f -
kubectl delete ns istio-system
Refarences
https://istio.io/latest/docs/setup/getting-started/#uninstall