The moment in Avengers: Endgame when Old Man Steve hands his shield, and therefore the mantle of Captain America, to Sam Wilson, is a hugely meaningful scene, but it’s caused a lot of confusion for fans. You see, Steve’s shield was destroyed during the final battle with Thanos. Clearly, though, he somehow got himself a new vibranium shield at some point as you can see this model is not the same as the old one due to the different markings and paint job.
The question is, then, where did he get it? Well, a new fan theory suggests one option, based on an off-hand mention contained in another MCU movie. Reddit user u/ak2sup has posted to the r/FanTheories subreddit arguing that the shield old Steve gave to Falcon was the same one mentioned by Happy Hogan in Spider-Man: Homecoming.
In that film, Hogan is in charge of emptying out the Avengers Tower of all Tony Stark’s experiments and prototypes, including an unseen new shield for Cap. This is presumably packed away with everything else on the Stark plane that’s then brought down in the subsequent fight between Spider-Man and Vulture.
The Redditor wonders if old Steve retrieved the shield from the crash site in Coney Island and that’s the one he passed onto Sam. Homecoming is set in 2016, but the theorist suggests that Steve holds onto the shield for years until 2023 to present it to his friend at the right moment.
Whether you buy into this theory or not probably depends on if you believe Steve has always been in the prime timeline and didn’t create an alternate parallel one when he went back to the 1940s to be with Peggy. While Avengers: Endgame directors the Russos prefer the latter explanation, writers Christopher Markus and Stephen McFeely go with the former. In fact, they’ve decreed that Old Steve attended his wife’s funeral in Captain America: Civil War. So, according to that reading, Old Steve was definitely active in 2016 and as such, could have somehow retrieved Stark’s second shield.