You can now search for products based on when they were last updated on Shopify. We've supported searching on the created date for a while but held off on adding the updated date because the results can be counter-intuitive.
The updated date for a product represents the last time any of the fields relating to the product, including inventory, were changed. Inventory levels can change when the product is ordered which means a popular product could have an updated date that's more recent than expected.
Despite this, searching for Shopify products by the updated date should be useful for identifying old products on your store.