Update [Wed 27th Dec, 2023 05:30 GMT]:
Pretendo has provided a new update, confirming the issues occurring were "indeed unintentional". While not all issues have been resolved, it appears new users on both 3DS and Wii U can now connect to servers.
"We noted in our last blog post that it was unclear whether these issues were intentional or not, and it appears that this was indeed unintentional. While we cannot confirm whether or not Nintendo will take action on all issues presented to them, we would like to thank Nintendo for taking the time to at least acknowledge the issues and look into correcting them. We would also like to specifically thank the engineers and other IT staff at Nintendo working to maintain these legacy servers despite the impending shutdown."
"In our last blog post we went into detail about how some parts of Nintendo's server architecture works, and how this led to new users on both platforms not being able to connect to game servers anymore. We are happy to announce that as of December 26th, 2023 at around 5:00 PM UTC this seems to have been corrected. We have verified this with several people internally, on both platforms and in several games. New accounts may now go online as normal."
You can get the full rundown about the latest Nintendo Network issues in a new blog update on the Pretendo website. This includes a detailed update about Super Mario Maker servers.
Original story [Sun 24th Dec, 2023 23:55 GMT]:
Earlier this year, Nintendo announced it would be ending "online play and other functionality" that uses online communication for both 3DS and Wii U as of "early April" 2024.
Now, in an update, it's been discovered that new users can't go online in any games on 3DS or Wii U. This information comes from a lengthy blog post on Pretendo, which mentions how this change took place "some time in late December 2023". It's not known if this is intentional or not.
If you try to play online with a new Nintendo account from here on out, it will reportedly result in 'Error code 106-0303' on Wii U and 'Error Code 006-0303' on 3DS. This is due to new accounts "no longer being synchronized" with servers.
Super Mario Maker players have also been encountering 'Error Code 106-0502' in "early December", but some existing users are still able to connect to the game's servers it seems. You can get a more detailed rundown about this on Pretendo.