Nintendo has confirmed their intention to bring the curtains down on online functionality for many of the games on their Wii and DS/DSi consoles. A full list of affected titles can be seen here.
Online play for 3DS and Wii U remains unaffected, with Nintendo citing the need to ready themselves ahead of the launch of games with online features, such as Mario Kart 8 and Super Smash Bros. A Nintendo spokesperson told Polygon that the "global termination" will "take place on 20th May 2014 Worldwide," before later adding the following statement:
As of May 20, 2014, certain online functionality, including online play, matchmaking and leaderboards for many Wii, Nintendo DS and Nintendo DSi games, will no longer be accessible. For a list of Nintendo games and services that are affected, please check https://www.nintendo.com. Users can still play these games in offline mode, which continues to offer a rich game-play experience. Other online functionality on Wii and Nintendo DS, such as access to the Wii Shop Channel, the Nintendo DSi Shop and video-on-demand services, are also not affected at this time.
Online play for Wii U and Nintendo 3DS games will be unaffected (aside from the Wii mode on Wii U and Nintendo DS games played on Nintendo 3DS family systems). Our focus now shifts to readying our Wii U and Nintendo 3DS matchmaking services for upcoming games such as Mario Kart 8 and Super Smash Bros.
We at Nintendo sincerely thank our fans for their continued support of our company's legacy systems. Your enthusiasm for games made for these systems speaks to their longevity, and the passion of Nintendo fans.
According to this press release by Nintendo of Australia, the Wii Shop Channel and Nintendo DSi Shop will not be affected, and will remain in place past May 20 when the Nintendo Wi-Fi Connection is shutdown.