Forecast report provides you with an option to check out the Overstocked products.

In this report Layout, OverStock quantity is the inventory left over after the planning period (lead time + days of stock period). If lead time is 7 days and days of stock is 7 days then any surplus inventory after accounting for 14 days of sales is the overstock quantity.
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Overstock value is the cost price of those overstock units if you've added the cost price to the item in Shopify.