Based off a question in the forum, I couldn't find any reference to splunk `indexKey` or `indexName` in the splunk clf at https://docs.openshift.com/container-platform/4.15/logging/log_collection_forwarding/configuring-log-forwarding.html#logging-forward-splunk_configuring-log-forwarding

Here's the addition needed:  https://github.com/openshift/cluster-logging-operator/blob/release-5.9/docs/features/logforwarding/custom-tenants/custom-tenant-support.adoc#splunk