Loki stack can be used in a custom way and custom tenants can be used in such cases hence Loki operator do not reject any of the below configuration 
mode:
default: openshift-logging
description: Mode defines the mode in which lokistack-gateway
component will be configured.
enum:

• static
• dynamic
• openshift-logging

Custom tentants are not supported when Loki is used for openshift-logging 
only supported way is to use : mode: openshift-logging
which will by default have 3 tenants audit, infrastructure, application

This Issue is created to add this note in Logging docs for more clarity