Did Bucky know of Steve's decision in Endgame?

Question

So, we know that in Avengers: Endgame

Steve decided to stay in the past to grow old with Peggy, thus making the choice to bypass his time stamp and, as a result, grow old.

In the few moments before he

Went back in time to return the Stones,

Bucky tells Cap:

"I'll miss you buddy," and Steve replies: "Try not to do anything stupid while I'm gone." At that moment Sam asks "How long will this take?" and Bruce replies (something like): "For us, 5 seconds. For him, as long as he needs to return the Stones."

I'm assuming here that Bucky didn't know that, until Bruce explicitly said so, that it would take only 5 seconds for Steve to get back. I'll miss you buddy would make a lot of sense if Bucky thought Steve would take days/weeks to get back (he has to return the Soul Stone to Vormir [somehow]). His look of surprise at seeing old Steve at first also made me wonder. (Though, who wouldn't be at least a little bit surprised seeing your best friend age 30 years in a matter of seconds?)

So my question is:

Do we actually know if Bucky already knew what Steve was going to do at the end, as (seemingly) implied by the narrative?

Answer 1

Having watched the scene a few times I think it’s quite telling in that Bucky knew. His whole demeanour throughout the scene and conversation with Steve is like he knew what was going to happen. Then when Steve doesn’t come back instead of panicking like the others he slowly turns to the bench where Old Cap is sitting and calmly calls to Sam.

We now also have in universe confirmation that he knew. In The Falcon and the Winter Soldier Bucky and Sam talk about Steve handing the shield over to Sam. Bucky remarks about talking about it with Steve meaning he knew the plan to give the shield to Sam and so he likely knew the whole plan.

Bucky: When Steve told me what he was planning, I don't think we understood what it felt like for a Black man to be handed the shield.

_{The Falcon and the Winter Soldier, Season 1 Episode 1, “Truth”}

It has also been confirmed by the Russo brothers that Bucky knew.

Also, they confirm — Bucky knew. When Cap was preparing to for the trip, which is only supposed to last a few seconds in the main timeline, his old friend from Brooklyn gives him a surprisingly heavy farewell.

Somehow, he was aware that Cap was going to live in the past, and it’s probably more than just intuition. “Especially when he says goodbye,” Joe explained. “He says, ‘I’ll miss you.’ Clearly he knows something.”

But how? Has Winter Soldier already met with Old Cap at some previous point? It seems the answer is yes.

On the other hand, Joe adds, “Sam doesn’t know something.” Falcon has no idea about Old Cap, which is why The Winter Soldier urges him to go up and talk to the now-elderly Steve Rogers. Bucky already has the answer to the questions Sam is going to ask.

_{Entertainment Weekly, Avengers: Endgame directors answer Captain America mystery}

Answer 2

Based on my review of the scene, Bucky knew that Cap won't return right away or may be won't ever return, let me explain : just after Bruce sent him to the past, Sam and Bruce are scared, Sam shout at Bruce and tell him get him back, in that moment Bucky turns around and smiles before seeing Old Cap, and when he sees him, he is surprised and immediatly after smiles for the second time and call Sam and tells sam to go, so I think that he knew, either Cap told him his plan or he knew Cap enough to tell that he won't be back, and that's why he told him that he'll miss him.