Before heading into the Quantum Realm, Steve and Bucky had a reversed back-and-forth of their interaction in Captain America: The First Avenger (“Don’t do anything stupid until I come back.”
So now that the Captain America of it all has been addressed, let’s go back to Bucky Barnes’ overall role in Captain America’s final Avengers: Endgame scene.
You’re taking all the stupid with you.”), but Bucky also seemed to know that Steve didn’t plan on coming back, and when we spotted Old Man Steve at the nearby bench, Bucky allowed Sam to talk to him first.