What is the Confidential_ThirdWheel_Test game mode in private matches in Rocket League?

The Confidential_ThirdWheel_Test game mode was an April Fool's game mode, released on April 1, 2022, and can still be selected as a game mode in private matches. When selecting the game mode, this will be shown as "Confidential_ThirdWheel_Test." You can see the original blog post here: Warning To All Rocket League Players (Opens in a new tab)