Is Rancher Support only for production environments?

No. Rancher Support can be procured for any Rancher installation/environment for which customer wishes to get SLA-based assistance. In addition to production environments, it is very common for orgs to procure support subscriptions to cover other environments, such as their Dev/Test, Staging, Demo, Perf, PreProd, and Production Sandbox, for which they have their own SLAs to meet with their downstream customer users.