No, you do not need to ship back your unused sensors for disposal.
You can apply any FreeStyle Libre 2 sensors that have not passed their expiration date and use them for the duration of wear. The expiration date indicates the last day the sensor can be applied.
To check to see the expiration date or “Use by Date” of your sensors, please look at the bottom of the sensor pack. You will see an hourglass icon with the expiration date to the right. The date will be displayed as yyyy-mm-dd.