The Wii U 5.5.5 system update was released on March 1, 2021. This was released in Europe, North America & Japan.
Official USA changelog:
- Further improvements to overall system stability and other minor adjustments have been made to enhance the user experience
VersionData and the OS (OSv10) was updated.
5.5.4 -> 5.5.5
Almost all RPL files were updated, it's still unsure if there're other modifications than the build dates.
The IOSU image was also modified, only the IOS-NSEC section has been modified.
A SSL verification callback was updated (the function @ 0xE1006970), the reason is still unclear.