After the Guardians of the Multiverse returned to their different realities, Black Widow, who had come from the world Ultron destroyed, replaced herself in the reality that one of her variants had died in from What If…The World Lost Its Mightiest Heroes?
But then came the What If…? finale’s end credits scene focused on Captain Carter and the Black Widow from here world.
Aboard a ship back in Peggy’s reality, the pair discovered a Hydra Stomper, the one that her Steve Rogers used in What If… Captain Carter Were The First Avenger?
Someone is inside and who else could it be but him, presumably having been preserved for decades just as Captain America was in the ice in the main MCU reality.
source: express.co.uk