Pfft :
Wii last update was on June 2010 , 2 years before Wii U
Wii was one of the best selling consoles of Nintendo and a console with a big success , so if they launched the last update 2 years before Wii U on one of the biggest Nintendo success , its obvious they will not care about Wii U , that was a failure .
Wii U is dead already , they stopped their production , BOTW will be the last 1st party game , Wii U is not anymore on Nintendo website .
Its obvious they are more busy with Switch and they only care for 3DS because it was succesful . If they released an update for 3DS , its because they still care for the 3DS. Wii U last update was 1 year ago and Switch is in less than a month launched worldwide , you can already expect 5.5.1 being the last update.
And even if they launch 5.5.2 / 5.6.0 / 5.whatver , in the end , that could also be the last update and still , hackers will get over the new firmware and will be 5.5.1 again , so if they launch an update or not , Wii U is still gonna be hackable.