While Nintendo puts out a lot of multiplayer and competitive games, they usually take a very hands off approach as far as fostering a community goes. They’ll put out the games, and the content, and even respond to player feedback, but on the whole, Nintendo won’t organize tournaments, or attempt to foster any competitions themselves, unlike, say, Blizzard and Overwatch.
However, that is changing somewhat with Splatoon 2. Nintendo of Europe has set up a new website for players to be able to sign up for and participate in tournaments. You can find tournaments across all skill levels; the first first Splatoon 2 Community Cup for Europe is going to be held on January 13, and there are 128 openings available. If you are part of team, Nintendo also invites you to prepare in the official Discord server, or fill out a form otherwise, which will match you up with a team.
Nintendo also promises that there will be tournaments held on a monthly basis, and will be free, on a national and European level. So it seems like, even though the official content updates for the game have ended, they plan on supporting it and inculcating a competitive community for the game, for the foreseeable future.