The Mandalorian proved that it could keep big secrets going all the way back to Baby Yoda in the first season, but the show really outdid itself in Season 2 by managing to keep Mark Hamill’s return as Luke Skywalker from leaking.
In fact, they used a Clone Wars character to hide the return of Luke.
As far as most people knew before the big reveal, Plo Koon was the Jedi who arrived to save the day for Mando and Co.
Of course, it wasn’t by pure happenstance that Hamill’s appearance went unspoiled, as the team behind the scenes went to great lengths to keep the secret.