Mario Kart 8 Deluxe now requires game version 4.0.0.
Nintendo's 4.0.0 update landed on 1 September. We held off until someone had played a
full race on it against our servers, not just connected: this week Mario Maker's versus
mode connected, formed the lobby and started the match before crashing at the finish line,
so "it connects" isn't the same as "it works".
CTGP Deluxe players are not affected. That path still runs on base 3.0.3 with the verified
mod, and it has always been checked separately.
Smash Bros. Ultimate stays on 13.0.5 and Super Mario Maker 2 on 3.0.3, unchanged from v1.8.1.
Only one game version is supported at a time, on purpose: two different builds in the same
lobby desync. If you're still on Mario Kart 3.0.5, update the game.