Testut broke the welcome news by showing off a video of a Mario Kart DS online race on Threads.
As he explains to The Verge, the online features are powered by melonDS' online support, "which connects to 3rd-party Nintendo [Wi-Fi Connection] servers (such as Kaeru, Wimmfi, AltWFC, etc)."
This means over 10 million Delta users will be able to play with other Delta users, as well as players using authentic DS hardware connected to the same 3rd-party Nintendo WFC servers. All Nintendo DS games with Nintendo WFC are supported, according to Testut.
Given that the online servers for the DS were closed down a decade ago, this is welcome news for those of us who remember the good old days of Nintendo WFC. However, there's a bit of a wait here; Testut says Patreon supporters will benefit first via a beta version, adding that "we don't have a public release date yet."