Nintendo has just rolled out a bunch of updates for a number of games including ARMS. This title originally made its debut on the Switch in 2017 and is one of the newest IPs from Nintendo in recent years.
The last update was rolled out in September 2018 and made a number of balance adjustments. The latest one isn't quite as exciting. Like the latest update for Animal Crossing: New Horizons, Nintendo says "several issues have been addressed to improve the gameplay experience".
You can learn more about ARMS in our Nintendo Life review: