A move the game's creator describes as "unfortunate".
In a Nintendo Direct focused purely on the upcoming Kirby Air Riders for Nintendo Switch 2, creator Masahiro Sakurai revealed that the sequel to the Gamecube original would feature a second button function. In addition to the B button which is used for the game's drift mechanic (similarly to A in the original Kirby Air Ride), Y now allows you to use a new charge ability.
The original Kirby Air Ride, released in 2003, famously only made use of a single button. The player vehicle would move forward automatically while A could be used to brake and drift. During the direct Masahiro Sakurai jokingly stated, "We've added a second button, unfortunately".