Avengers: Endgame didn't explain how Captain America (Chris Evans) fixed his broken shield before handing it to Falcon (Anthony Mackie), but this could be enough proof that Steve Rogers crossed to a different timeline in the MCU. During Captain America's one-on-one fight with Thanos (Josh Brolin) during the movie's climactic battle, the villain repeatedly hacked it with his sword resulting in the iconic shield getting fragmented. Marvel never showed it being fixed; however, by the time he re-emerged at the end of the movie, he miraculously had it again in one piece, thus raising questions some questions.
After his falling out with Tony Stark (Robert Downey, Jr.) in Captain America: Civil War, the super soldier was forced to leave his iconic weapon behind. It wasn't until after the five-year time jump in Endgame that Steve was reunited with his shield, which ultimately broke during the film's final battle. Once Thanos was defeated, he had one last mission - return all six Infinity Stones to their original times, as Smart Hulk (Mark Ruffalo) promised the Ancient One, so as not to create alternate timelines. Using the additional Pym particles he smuggled and a new Quantum Tunnel mechanism, he accomplished what needed to be done, but when he returned to the primary MCU timeline, he was much older. He revealed that he lived a full life with Peggy (Hayley Atwell) after returning all the gems and only came back to hand off the shield. The problem is, no one knows how he was able to fix the broken Captain America shield.
Endgame didn't reveal the specifics of how Captain America repaired the shield he handed to Sam. That said, the existence of a fixed shield lends credence to the idea that Steve's decision to be with Peggy resulted in an alternate reality. This is backed by what's been established regarding how time travel works in the MCU as well as some physical details seen on Falcon's new shield, such as a line on the silver circle.
The whole Captain America ending spawned a string of questions. Directors Joe and Anthony Russo have conflicting answers with writers Christopher Markus and Stephen McFeely. The former believe that Steve created a new timeline when he decided to stay with Peggy, while the latter theorize that it's essentially all part of the same MCU continuity. The writers' argument insinuates that there had been two Steve Rogers in the MCU all this time. However, based on the time travel rules laid out in Endgame, Steve deciding to return to the 1940s and stay with Peggy would've created a branched timeline - similar to how Loki escaping with the Tesseract during the Battle of New York also created a different reality.
In fact, Steve didn't even have the broken shield when he traveled back in time - only taking the six Infinity Stones and the Mjolnir with him to return to their original settings. Given this, there was no way to fix his shield in the same continuity unless he traversed an alternate reality where a different shield was existing. It's worth noting that while the shield he gave to Sam looked identical to the original, they seemed to be different. Instead of a clean silver layer as part of its stripes, there's a noticeable thin stripe in its middle. Sam's version has pronounced star cutouts which the original version didn't have as well. There's no physical sign that the shattered one was simply repaired, meaning, this might very well be an entirely different shield - something he might have asked Howard Stark or T'Challa to created with Sam specifically in his mind as its wielder.
Avengers: Endgame filmmakers decided to sacrifice narrative clarity when it came to the timeline specifics for sentimental reasons for Captain America's final fate. Considering the response from the public, the trade-off was ultimately worth it. Marvel Studios has yet to address this pressing issue, but that doesn't mean they won't clarify these lingering questions in The Falcon and the Winter Soldier.
