The operator creates a number of resources, which are not managed by OLM (OperatorHub) and need to be removed manually after uninstalling the Istio operator. As cluster-admin execute the following commands (where operator-namespace is the namespace within which the Istio operator was installed, typically openshift-operators):

$ oc delete validatingwebhookconfiguration/<operator-namespace>.servicemesh-resources.maistra.io
$ oc delete -n <operator-namespace> daemonset/istio-node
$ oc delete clusterrole/istio-admin
$ oc get crds -o name | grep '.*\.istio\.io' |xargs -r -n 1 oc delete
$ oc get crds -o name | grep '.*\.maistra\.io' |xargs -r -n 1 oc delete