Nintendo has announced that Wii U and 3DS titles will no longer be playable online as of April 2024, limiting all games to offline play only.

The details:

Nintendo has posted a new announcement on its support sites, confirming that "online play and other functionality that uses online communication" will be shut down in "early April 2024" for both the Wii U and 3DS. This includes all online multiplayer modes, leaderboards, and other in-game functions.
However, the company confirmed that players will be able to "download update data and redownload purchased software and downloadable content from Nintendo eShop" even after the shutdown, though this may change in the future.
Although the news of a shutdown was expected, fans are still mourning the loss of online connectivity, as games like Splatoon and even Animal Crossing: New Leaf place a focus on competing against or visiting other players all over the world. Nintendo has stated that Pokémon Bank will remain operational for the foreseeable future as players continue to use it to transfer Pokémon between games.
Nintendo shut down the eShops for both systems earlier this year, rendering over a thousand digital-only exclusive titles inaccessible. This prompted an impressive preservation effort by YouTuber Jirard "The Completionist" Khalil, who spent over $22,000 over a year purchasing and downloading every game available on the service before its closure.

More gaming news:

Nintendo has confirmed that it will continue to support the Switch through 2025, with the console remaining a priority until the end of the fiscal year 2023-24. This comes after several reports and rumors that the company intends to release a new console in the second half of 2024.
After fans spotted that the reveal trailer for the Star Wars: Knights of the Old Republic remake had been made private, Sony responded by stating that the video had simply been removed due to the expiration of the license to the music used.

Nintendo is shutting down online play for the Wii U and 3DS in April 2024