It’s honestly been surprising to see that Nintendo hasn’t announced Super Smash Bros on Nintendo Switch. Everybody expected that they’d release the Wii U/3D version on Switch ALA Mario Kart 8 and most other Nintendo Wii U games, and there would have been nothing wrong with that.
Long-time Nintendo insider Emily Rodgers has stated on Resetera that Super Smash Bros will be released on Nintendo Switch later this year with existing Wii U/3DS content as well as brand new content.
In all honestly, this is exactly what I’d have predicted. I’d suspect the reason why this one hasn’t released yet is because Nintendo are adding some news stages, new characters and a new mode or too. It’ll also probably launch alongside their proper online service.