Will demand forecasting work for a brand-new SKU with no history?

Not directly. A brand-new SKU has zero data points; Prophet can’t fit a model. The pattern that works: category-level forecast with an analog-SKU scale factor. (1) Forecast aggregate daily demand for the category (jackets, hoodies, whatever) at the category level using all available history. (2) Pick 2-3 “analog” SKUs with similar price + size + style that have at least 12 weeks of history. (3) Compute the analog SKUs’ share of category demand. (4) Scale the category forecast by that share for the new SKU. After 60-90 days the new SKU has enough data to switch to its own Prophet model. Re-evaluate weekly.