The latest software version is required to enter Online mode.
Local wireless play is not compatible with Ver. 11.0.1 and earlier. All players who want to fight in local wireless battles need to make sure they have the same software version.
There may be compatibility issues with replays from Ver. 9.0.0 to 11.0.1. Replays from Ver. 8.1.0 and earlier are not compatible. If you want to keep your replays, convert them into videos before installing the update. You can do this via Vault → Replays → Replay Data → Convert to Video.
Super Smash Bros. Ultimate is available now on Nintendo Switch. For more information regarding this patch, visit the official Nintendo site.