see https://github.com/rancher/rancher/issues/42191#issuecomment-1657880025

delete_stale_resources.sh

#!/usr/bin/env bash
namespace=${1-fleet-default}
delete_before_timestamp=${2-2000-01-01T00:00:00Z}

jsonPath='{range .items[*]}{@.metadata.creationTimestamp}{"_"}{@.metadata.name}{"_"}{@.spec.clusterLabels.management\.cattle\.io/cluster-name}{"\n"}{end}'
cluster_regs=$(kubectl get clusterregistration -o=jsonpath="$jsonPath" -n $namespace | sort)
read -ra regs -d '' <<< "${cluster_regs}"

last_idx=$(( ${#regs[@]} - 1 ))
for (( i = 0; i < $last_idx; i+= 1 )); do
    IFS=_ read -r creation_timestamp name cluster_name <<< "${regs[i]}"
    if [[ "$creation_timestamp" < "$delete_before_timestamp" ]]; then
        kubectl delete --ignore-not-found=true clusterregistration "$name" -n $namespace
    else
        echo "KEEP $name $creation_timestamp $cluster_name"
    fi
done