Interesting, but I initially expected this to be about the unusual opening employed to victory by Magnus Carlson against Kacper Piorun on May 7, 2024[1] (1. a4 e5 2. Ra3).
It's even more interesting because an unknown IRL Chess.com player named Viih_Sou (since revealed to be Brandon Jacobson[2][3]) used this opening to defeat Daniel Naroditsky on May 2, 2024[4] only to be subsequently banned for violating the Fair Play Policy[5].
Why would that be the rarest chess move? It's a pretty common way to try to start a chess game, so much so that kids have to be taught not to do it because it's flat-out terrible.