I follow this stuff very closely, and it seems everyone missed this one. Steel Divers: Sub Wars has apparently suffered a casualty, being one of the first games to lose online after Nintendo were apparently “unable to restore” the functionality due to security vulnerabilities that were previously discovered.
“Starting on Sep. 4, 2023, we performed emergency maintenance for the Steel Diver: Sub Wars game, which released on Feb. 14, 2014 for the Nintendo 3DS family of systems. Unfortunately, we have made the difficult decision to discontinue the game’s online services after being unable to restore them.“
Worth noting that Nintendo’s online services for 3DS and Wii U do have a death sentence set for April 8, 2024. Pokémon Bank and Poké Transporter are unaffected for now. After the death date, Pretendo Network will be one of the only ways to play Wii U and 3DS games online.
(Disclosure: I am materially connected to people on the Pretendo Network project, and have access to Beta servers, but they did not have any say in or knowledge of this post prior to publication.)