Black Widow's death was one of the more heart-wrenching moments in Avengers: Endgame, but it almost went very differently.
2019's Avengers: Endgame spelled the end of a lot of things in the MCU. Not just the end of a decade-long tussle with Thanos and his various sidekicks and minions, but also the end of a few long story arcs. Three heroes who had been a part of the MCU since the beginning (or close to it): Iron Man, Captain America, and Black Widow.
While Cap' had the privilege of simply aging away, Black Widow and Iron Man were erased for good by being killed off. While the movie will always be remembered for the emotional, heroic, and complete circle demise of Tony Stark, Natasha Romanov's death scene was also very hard to watch. However, there were plans for it to go very differently.
In the movie, Black Widow and Hawkeye travel to Voromir in order to retrieve the soul stone. Doing so requires a sacrifice, so the two battle to be the one who becomes that sacrifice. Eventually, Black Widow gets her way and dies so that Hawkeye can return with the stone. The alternate scene has the same ending but doesn't quite tug on the heartstrings as much as the one the movie's makers ended up going with.
In this version, Black Widow and Hawkeye appear to have arrived on Voromir at the same time as Thanos. The heroine takes a couple of shots from the super-villain's loyal foot soldiers before throwing herself off the cliff. The last thing she sees before doing so is Thanos closing in on Hawkeye. There's a blinding flash of white light as she hits the ground, so Hawkeye is presumably transported away from the scene, soul stone in hand, as has been the case whenever others have been to Voromir.
Ultimately, we're glad the original scene was the one that made the cut. It added to the emotion of the moment and also made a lot more sense. If the Avengers knew there was a chance Thanos would be at Voromir, why would they send two people with no powers? Also, this alternate scene looks more like Black Widow is committing suicide than Hawkeye having to let her go, hence the sacrifice part of the soul stone deal.