Azure Container Registry gives you the option to set a retention policy for stored image manifests that don’t have any associated tags (untagged manifests). When a retention policy is enabled, untagged manifests in the registry are automatically deleted after a number of days you set.
The following example sets a retention policy of 30 days for untagged manifests in the registry zigmax:
az acr config retention update --registry zigmax --status enabled --days 30 --type UntaggedManifests
You can also define the retention policy for un tagged manifests in the Azure Portal: