Nobody REALLY dies in the MCU (or in comics, in general). So, character deaths are only as real as the actors who play them allow them to be. For example, we all know that if Robert Downey Jr. wanted to be resurrected, they would probably do it. However, RDJ says he's done with the MCU. And it appears that Scarlett Johansson is too.
Johansson said in an interview with Fatherly that she "has no plans to return as" Natasha Romanoff. "I feel really satisfied with this film," she stated of Black Widow. "It feels like a great way to go out for this chapter of my Marvel identity." However, there may still be a role for Johansson at Marvel. She could direct, produce, or even write something. "I would love to be able to continue to collaborate with Marvel in other ways because I think there's just an incredible wealth of stories there," she said. "Re-imagining this genre is something that I find very interesting."
Kevin Feige also left the door open for Johansson in a recent interview with Entertainment Tonight. "Marvel is always about new beginnings and Scarlett Johansson is such an amazing partner for us," he elaborated. "She was a producer on [Black Widow]. She was the one that brought us our amazing director, Cate Shortland. I am excited to continue working with her in any way possible if we're so lucky,"
[via]