The Splatoon franchise established its esports tournament circuit in 2018.
That year marks the point identified by Wikipedia for the series’ own tournament circuit. By then, Splatoon had grown beyond a novel Nintendo multiplayer game into a competitive title with organized events, skilled teams, spectators, and regional championships. Its four-versus-four format made matches relatively easy to follow while leaving room for detailed team strategy.
The date can be confused with other important milestones. The original Splatoon launched in 2015, Splatoon 2 followed in 2017, and the Octo Expansion arrived in 2018. Those dates describe games or downloadable content, not the establishment of the franchise’s esports circuit.
Competitive Splatoon centers on coordinated ink coverage, positioning, weapon roles, and rapid movement through friendly ink. Although local and regional competitions existed around earlier games, 2018 is the canonical year given for the franchise’s own esports tournament circuit in the cited reference.