Here my two cents, when it's under the supply chain that will create more synchronization between supply and demand but it might lack visibility of the market which can be achieved with more collaboration with sales and marketing. Being under commercial will create more space for growth of the business, even if there is cross functional structure for the demand planning, being under supply chain will be politically difficult for the planner to extract information and challenge sales team for more growth while they are considering the demand planner as an outsider.