“In early April 2024, online play and other functionality that uses online communication will end for Nintendo 3DS and Wii U software,” Nintendo said on its Support website.
“This also includes online co-operative play, internet rankings, and data distribution.”
“if an event occurs that would make it difficult to continue online services for Nintendo 3DS and Wii U software, we may have to discontinue services earlier than planned.”
As for Pokémon Bank
Pokémon Bank will still retain its online functionality (presumably since it’s still used by players and uses online connectivity as its central mechanic).
Edit: Official Announcement with Q&A: https://en-americas-support.nintendo.com/app/answers/detail/a_id/63227/~/announcement-of-discontinuation-of-online-services-for-nintendo-3ds-and-wii-u
For over a year it’s been fairly trivial to play unauthorized copies of games online with normal players. Should be no surprise they are pulling the plug after their copy protection is broken.
Should get about 6 more years out of the Nintendo Switch’s online service before they pull the plug too if this is any indication.