This is the paper I would normally refer to: https://www.vmware.com/content/dam/digitalmarketing/vmware/en/pdf/products/vsphere/vmware-vsphere-pricing-whitepaper.pdf
But there is no mention of licensing in the prerequisites here: Add a Serial Port to a Virtual Machine
Your question has appeared here before, but was unanswered: Minimum License for Network Serial Ports in vSphere 4.1?
I found this Cisco article which mentions Enterprise (no longer available) or Enterprise Plus licensing is required: Configure Serial Port Access to NX-OSv 9000 on VMware ESXi - Cisco
If that Cisco document is correct, that would be the “best” answer, and would explain why you had it working during your evaluation.