What you went through was the free introductory run of Battlegrounds and the quest to get the hammer so you could get through the umbral engram system for that first quest. You can buy previous seasons of the current year by buying the deluxe edition of the current main DLC. In this instance that would be the Deluxe Edition of Beyond Light. This would also mean that if you didn't own Season 12 you will receive its content as well. Past passes are not available, only the current pass is. Hope this clears up confusion