Today in Japan, Nintendo released a new system update for the Wii U. The last firmware, 5.5.3, was released in early September 2018.
Nintendo of America detailed the previous update as fixing the following:
Improvements to system stability and usability
Further improvements to overall system stability and other minor adjustments have been made to enhance the user experience.
The latest firmware update, 5.5.5, is currently only detailed on Nintendo’s Japanese support page, but covers the same thing as the previous one that’s mentioned above.