In "What If...The World Lost Its Mightiest Heroes?", we visited a reality where Hank Pym had been overcome with grief after the death of his daughter, Hope. She'd signed up to join S.H.I.E.L.D. years after her mother's death and was killed in action; Hank, meanwhile, suited up as Yellow Jacket and killed Iron Man, Thor, Hawkeye, The Hulk, and Black Widow to end the Avengers Initiative.
However, an exchange between Nick Fury and Hank in the closing moments of What If...? has revealed an Easter Egg that seemingly confirms who was responsible for Hope's death.
Noting that she died on a mission "outside Odessa, Ukraine," Fury doesn't elaborate on what happened beyond that, but some have remembered a conversation between Steve Rogers and Natasha Romanoff in Captain America: The Winter Soldier in which she recalled crossing paths with The Winter Soldier (a.k.a. Bucky Barnes) after he "shot out [her] tires near Odessa."
The HYDRA-controlled Bucky then "shot [his target] straight through [her]," and while Black Widow survived that encounter, Hope obviously wasn't quite so lucky.
This is an interesting twist, and it's a shame in some ways that this episode of What If...? didn't better explain the one event that changed this reality. However, doing so would have obviously spoiled the big reveal that Hank was the one doing the killing, so we can't fault the series too much!
Did you catch this Captain America: The Winter Soldier Easter Egg in What If...?