Even though all of the standalone movies in the MCU lead up to an Avengers movie, Captain America: Civil War became the focal point in Phase 3. Ever since its 2016 release, the subsequent movies - including Avengers: Infinity War - directly follow the aftermath of Civil War.
When Civil War was released, it ushered in a new wave of heroes, including Black Panther and Spider-Man. However, aside from growing the Avengers, it also tore them apart. Mixed feelings pertaining to the Sokovia Accords led to split alliances, and the ripple effect is still as potent as it was during the movie's climactic airport fight sequence involving an epic hero vs. hero Battle Royale. Thankfully, the recent Blu-ray release for Black Panther has helped answer some questions regarding why exactly the aftershock from Civil War is still so vital to the MCU's overarching plot.
Screen Rant interviewed Black Panther executive producer Nate Moore, who elaborated on the significance of Civil War's consequences pouring through all of Phase 3. He explained that from the get-go, the intention wasn't to have Civil War be quite as integral as it ultimately turned out. After the movie was complete, the main crux of the story divided Avengers, which in turn ignited "ramifications" that were more severe than expected. It ended up inspiring new angles and ideas that weren't previously considered, such as how Ant-Man & the Wasp would begin, considering that Scott Lang was imprisoned at the end of Civil War. Luckily, though, Marvel Studios' creative team were given plenty of time to roll with the punches, so to speak. And what's more is that, despite how intricately planned the MCU might seem, Moore explained that there was no "grand plan that when we started Civil War we knew it was going to do X-Y-Z." It just so happened that the events of Civil War were so consequential that the aftershock into subsequent films was inevitable.
As for Black Panther, specifically, Moore explained that Civil War "threw [director] Ryan [Coogler] for a loop," as he had originally intended to adapt the story closer to the comics, with King T'Chaka dying much earlier in T'Challa's life. So, instead of being someone who had roughly 20 years to prepare for the throne, the movie version of T'Challa is "coming into power for the first time." This forced Coogler to approach his adaptation from a new angle, which ultimately resulted in a more complicated character.
In a lot of ways, Captain America: Civil War (or "Avengers 2.5," as it's joking referred to as) marked the beginning of the end for certain narrative threads within the MCU. And once the door closes on Phase 3 with next year's Avengers 4, audiences may well see the MCU clean house on its original heroes, following a domino effect ushered in as early as Civil War.
Black Panther is now available on VOD, Blu-Ray, and DVD.