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 no longer 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".
If you try to play online with a 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 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.