Are you referring to the browser extension/plugin that allows you to claim some of the rewards from past seasons? This uses the bungie api to claim these, but bungie only officially let's you claim this seasons and the last via their website. At any time, the bungie api can be updated to remove the methods the extension/plugin uses to claim season items from further back than they officially support. Bungie can (and have been before) remove the api calls for seasons further back than the last, making them completely unattainable. This is why quite a few seasons are not available to grab anything unlocked but not claimed on a lot of the past seasons you can select on the extension/plugin. As this is not an official way to get items further than a season ago, there's nothing you can do unfortunately.