This is according to an announcement by Nintendo (via Game Rant) in which it was revealed that Nintendo will be hosting a competition during E3 2018 in which players will be able to try out Super Smash Bros. on the Switch. This seems to suggest that the game won’t be available before then, and with E3 2018 taking place in the middle of the year, we guess this means that Super Smash Bros. will be released after.
https://twitter.com/NintendoVS/status/976790642314522624
At the moment save for the teaser trailer that Nintendo released for the game, not much else is known about it. We know that the game will introduce new characters to the roster from the Splatoon franchise, but whether or not Super Smash Bros. will simply be an updated port of the Wii U version of the game, or if it could be a brand new title developed from ground up (we’re hoping it’s the latter).
However in the meantime it seems that Microsoft also wants in on Super Smash Bros. and has recently expressed their interest in letting the Banjo and Kazooie characters be part of the franchise.