Avengers: Endgame ended with Captain America as an old man bestowing his shield to The Falcon -- after living a full life with his wife Peggy Carter. That scene launched numerous fan debates, wondering if Captain America returned to the movie's main timeline or not.
Directors Joe and Anthony Russo shared their view on Old Man Cap's return on Lights Camera Barstool. Joe Russo said, "He would have been in a branch reality and then had to have shifted over to this, so jumped from one to the other and handed the shield off."
In Avengers: Endgame, Captain America went to the past to place the Infinity Stones where they belonged in time and was expected to return immediately. However, after his completed mission, he appeared as an elderly man. Cap explained to The Falcon that he took the opportunity to time-travel to connect with Carter, marrying her and living a life with her. The revelation confused Falcon who was unsure how Cap ended up in his timeline.
Russo added, "One thing that's clear that Anthony and I have discussed -- I don't know that we've discussed this publicly at all -- Cap would have had to have traveled back to the main timeline. That's something that, yes, he would have been in a branch reality, but he would have to travel back to the main timeline to give that shield to Sam Wilson."
The Russo's clarification confirmed that Cap used his last chance to return to the main Endgame timeline, but waited to do so until he lived a life with Peggy Carter. However, the brothers didn't confirm why Cap chose that moment to leave the branch reality.
Source: Lights Camera Barstool via Collider