With the ongoing burial of Wii U, Nintendo has announced on Splatoon’s official tumblr (link) that SplatNet for Original Wii U Splatoon will cease its service on 30 September 2017
This does not mean that players can no longer play Splatoon multiplayer online. SplatNet is an online service which allows users to track their stats, weapon usage, and upcoming maps.
[Notice] The SplatNet service for the Splatoon game on the Wii U system will be permanently closed on September 30th, 2017. Online multiplayer battles will still be available, as this update only affects the SplatNet service. We sincerely thank all players for using this companion site for the original Splatoon game.
While the nails on Wii U’s coffin has been nailed with the launch of the Switch, the slow shutting of its online services will give the lacklustre console its burial.