Nintendo's Japanese support service has announced that it will be ending repair support for the Wii U. When exactly? Well, that really depends on when its current parts inventory runs out.
The announcement was made on the Japanese Nintendo Support Page, confirming that the period of holding repair parts for the Wii U has now passed. As such, customers requiring a repair will need to book in sooner rather than later. Nintendo is unable to provide guidance as to when the repair support will end as this is contingent on remaining stock.
Regarding accessories for the Wii U (i.e. Gamepad, etc), repairs for these will cease at the same time as the main console unit.
For those in the West, Wii U repairs have already ceased, with the only options available on the Nintendo Support page being for the Switch, Joy-Con, 3DS consoles, Game & Watch (modern versions), Mario Kart Live, and the NES and SNES Classic Minis.