Captain America's time-traveling journey is somewhat ambiguous, but Peggy Carter (Hayley Atwell), living out the life stolen from him when he became frozen in ice for 70 years. Old Captain America then shows up in Endgame, ing on his shield to Sam Wilson (Anthony Mackie), who becomes the new Captain America.
Endgame adds mystery to Captain America's mission to the past. The movie reveals that traveling to the past doesn't affect the main MCU timeline. Instead, it creates an alternate reality. However, Old Cap showing up in Endgame confuses things, and even the movie's creators can't agree on what happened. According to the Endgame directors, Joe and Anthony Russo, Cap created a new timeline when he traveled back to be with Peggy Carter, and he had to travel again to the main MCU timeline to give his shield to Sam. However, the Captain America: The Winter Soldier.
Black Widow may finally settle what's going on with Captain America. In a prison scene, Red Guardian (David Harbour) arm wrestles against other prisoners and tells stories of his glorious past, claiming he fought Cap in the '80s. Another prisoner questions the story, saying Captain America would've still been in the ice in the '80s, and Red Guardian responds by breaking his wrist. While Marvel plays the scene for humor, the moment may also reveal a secret twist. It's possible that Old Captain America lived in the main MCU timeline.
Red Guardian's story seems to shed light on Captain America's time traveling. Though Red Guardian doesn't seem to have a problem getting hyperbolic when it comes to storytelling, he mentions his past with Cap multiple times in Black Widow, hinting that he's telling the truth. If Cap did travel to the past of the main MCU timeline, he could've still carried out some missions while laying low. Though his younger self was in the ice in the '80s, the older Cap could've still fought Red Guardian. He just would've been around his 70s at the time. Since Cap is a super soldier, his age probably didn't slow him down much.
Of course, there could be other explanations for Red Guardian's story. In John Walker (Wyatt Russell), showing the country doesn't have any qualms with making new Captain Americas. Also, the show reveals there were other superpowered soldiers, such as Isaiah Bradley (Carl Lumbly). Because of these other superhumans, it's possible Red Guardian fought a different iteration of Captain America. Then again, as Red Guardian's fellow prisoner in Black Widow suggests, he could be making up the story. When it comes to creating mystery around Captain America, Marvel could do it all day.
