Yes, I know that Mario Kart Tour, the mobile version of Mario Kart, was announced a while ago, but at the time it felt like FOREVER in the future. Now that we're in 2019, we're so dang close tot he release!!!
Technically we don't have an exact release date, but according to a tweet from January 2018, it will release sometime before the end of March 2019.
Details about the mobile version of the game are pretty slim and basically non-existent. Basically all we know right now is that use of the game will be "free to start".