The plot twist regarding the death of Tony Stark's (Robert Downey Jr.) parents was plotted as early as Iron Man. As the inaugural hero of the Marvel Cinematic Universe, the genius, billionaire's origin movie was mostly standalone, but with Kevin Feige visualizing what the franchise could be, it planted several plot threads in the movie that eventually paid off.
Howard and Maria Stark's death wasn't much of a focus in Iron Man, but it was mentioned during the montage about Tony. Aside from confirming how the Starks died in a freak car crash, there wasn't much mention about it in the movie. Instead, director Jon Favreau opted to center the movie on the titular character's hero journey which arguably its best aspect. However, it didn't take long before we learn more about them, particularly Captain America: Civil War revisited the manner of their death with the reveal of what truly happened to them on that fateful December night.
Aside from the Avengers: Infinity War.
It's safe to assume that the specific details with regard to the plot twist weren't in the MCU cards while they were doing Iron Man considering that it was too far out in the future, but they did plant plot seeds in the 2008 film that made it easier to execute the unexpected narrative turn. As seen in the archived front page clippings in the aftermath of the Starks' death in Iron Man (and also Captain America: The Winter Soldier), the date of their publishing is December 17, 1991 - a day after their actual death. In Civil War, Baron Zemo (Daniel Brühl) repeatedly asked Bucky for the mission report for December 16, 1991, which established flawless continuity between the two films despite a significantly long gap in release dates.
Regardless of how Marvel Studios actually planned this, it's impressive how they're able to take a minute detail from their very first film, Iron Man and use it as a catalyst for one of their most important MCU subplot. Bucky having to do with Starks' death effectively made Tony and Steve's conflict personal rather than professional, which makes their two-year estrangement more believable. ittedly, the superhero franchise's continuity isn't perfect. Despite this, they're still the best when it comes to sustaining an interconnected narrative thanks to their ability to play the long game and maneuver their story to fit in with what they've previously established.
