If you aren't playing Destiny 2, here are other popular games that had a "Season 15" in or around 2021:
: Reach the pinnacle power cap (1320-1330 for that season) to maximize the effectiveness of high-level drops.
: A 6-player matchmade activity set in the Dreaming City.
: Multiple rotations occurred, featuring specific quests like "For the War to Come" which required zone captures and pulse rifle kills.