Both the original Splatoon and Mario Kart 8 on the Nintendo WiiU have been temporarily taken offline by Nintendo.
The developer's website plainly states that "urgent maintenance [is] required to fix a vulnerability related to online play." At time of writing, Nintendo hasn't provided a window for when the issue will be resolved.
In the case of Splatoon 1, this is particularly notable since the WiiU is currently the only way to play the colorful shooter. Mario Kart 8 was ported to the Nintendo Switch in 2017 and recently released new DLC.
OatmealDome, a developer for Nintendo-specific emulation, speculated that the vulnerability issue in question is ENLBufferPwn. The exploit would allow someone to take over another player's console just by playing a multiplayer game with them.
Nintendo was informed of the vulnerability in 2021, and towards the end of 2022, released an update to Nintendo 3DS and WiiU to fix that exploit. At the time, the updated games included Super Mario Maker 2, Splatoon 2 and Mario Kart 7.
Whether or not that specific exploit is the culprit, it was noted at the time (and by OatmealDome now) that neither Splatoon and Mario Kart 8 had received similar update, despite it reportedly being known how vulnerable both games were.