StormClan is canon in Warrior Cats through the Dawn of the Clans backstory and related lore, where it appears as a combined clan formed by ThunderClan and WindClan descendants in the early history of the series. The clearest public reference I found describes StormClan as part of that early- clan history rather than a later main-series addition, and it notes that the clan eventually faded from memory within the story world.

What that means

  • It is not one of the modern “five main Clans” in the usual present-day setting.
  • It belongs to the ancient/history layer of canon tied to the founding-era timeline.
  • So if you mean “when was it made official by the franchise,” the answer is that it became canon when that founding-era lore was established in the series’ backstory material, not as a separate present-day Clan.

Short version

StormClan is canon as part of the ancient Warriors timeline , especially the backstory around the earliest Clan era.

TL;DR

StormClan isn’t a current main Clan; it’s canon through the series’ early- history lore and backstory material.