Deactivating a plugin would generally reduce your subscription amount for your next billing cycle. The reduction in amount will depend upon your choice of edition, number of plugin licenses, and the billing cycle that you have opted for. There will be no change in your subscription amount in the current billing cycle if you are only deactivating the Kitchen Display plugin AND not updating your store / POS count. Kitchen Display Plugin is license based. The amount that you pay for the plugin depends on the number of Kitchen Display licenses that you opt for.
Note: Plugins in Tagrain are either Subscription Based or License Based. So the price that you pay for your plugins is based on the number of POS terminals that you have in your setup and the number of licenses that you have acquired.