Unchecking the Active tickbox on a Product marks it as inactive, but this doesn't function as a true "discontinue this product" switch the way you might expect.
The Active field is a standard checkbox field on the Products module, intended to indicate whether a product is currently offered. It doesn't delete the product, and existing Quotes, Deals, or Sales Orders that already reference it are completely unaffected, since the historical record of what was sold remains intact regardless of a product's current active status.
Since the Active field alone isn't dependable for this, consider layering on a Validation Rule or a Layout Rule that blocks a Deal or Quote from being saved if a discontinued product is selected, or renaming discontinued products clearly, such as prefixing the name with "DISCONTINUED", so reps at least see a clear visual warning in the picklist even if the item technically remains selectable.
Deleting a product that's already used in existing Quotes or Sales Orders isn't straightforward either, since Zoho CRM requires it to be disassociated from every related record first. If you need to see every place a product is used before attempting this, a report is currently the most reliable way to find all its associations in one place.