Warzone uses Blizzard's Battle online gaming servers. Other games that use those servers include Overwatch, World of Warcraft and StarCraft 2. This means that you don't need to have a Steam account in order to play the game, but rather you need a Battle account.
Valve's Steam service is a must-have for any PC gamer. Its excellent selection, recommendation features, and deals make it one of the first applications to install on any gaming PC. On Steam Deck, it's essential.