Reference sub-workflows are currently unsupported. Use reference launch plans instead.

Is there any reason the reference sub-workflows are currently unsupported and if it is not supported then in what scenario we use reference workflow?

AFAICT reference subworkflows are not supported because of the "cost" of network calls to flyteadmin to populate the WorkflowTemplate object which comes from the parent workflow. Instead, you can use reference LaunchPlans to point to workflows that are already registered, you can even invoke a specific version. https://docs.flyte.org/en/latest/user_guide/productionizing/reference_launch_plans.html#reference-launch-plans