If Steve Rogers, as Avengers: Endgame has shown, never went on the ice to go to the future to form the Avengers, then the current timeline doesn't make sense. Also if he went back before he went on the ice, then what happened to the other Captain America in general?
If he went back after the past Cap went on ice then that would mean that there are now inconsistencies with Winter Soldier and Civil War where Peggy is mentioned as to have already lived a life without him.