Last year, it was confirmed that Frank Grillo would be reprising his role as Brock Rumlow/Crossbones in Avengers 4. However, it was unclear at the time how his character will be utilized after the story. His last appearance was in Captain America: Civil War, in which he killed himself with explosives, so there were rumors that he actually survived the explosion for the upcoming Avengers film. Thanks to Grillo’s recent appearance on the UFC Unfiltered podcast with Jim Norton & Matt Serra, it is now clear that Crossbones will only be seen in the movie through a flashback.
Roughly nineteen minutes into the most recent installment of UFC Unfiltered, the hosts talk about the character of Crossbones and ask Grillo why the filmmakers decided to kill him off. Grillo does not go into detail behind the decision, but he does reveal that Civil War would not be the last movie with Crossbones by revealing new information about Avengers 4:
“He makes an appearance in the next Avengers movie, but it’s a flashback.”
In addition, Grillo reveals that he is allowed to speak up about the film because he has no contract with Marvel Studios and does not plan to be in an MCU movie again. This suggests that the flashback is most likely footage from Civil War or The Winter Soldier, and that it will not serve as a major part of the movie. Still, fans of the character will be able to anticipate his brief cameo in the next Avengers movie, even if said cameo will not be new material.
Source: UFC Unfiltered
Avengers 4 is scheduled for release on May 3, 2019.