According to Avengers: Endgame director has confirmed that Black Widow is gone forever. Any hopes of her returning forward down the timeline has been shattered.
Avengers: Endgame was the saddest of all the Marvel films as it saw the end of some of our favorite Avengers. The first real casualty in the film was Black Widow who sacrificed herself to acquire the Soul Stone. The ultimatum set by Red Skull was that in order to get the soul stone "you must lose that which you love. A soul for a soul…”
Thanos first sacrificed his beloved "adopted" daughter Gamora for the soul stone in Infinity War and in Endgame Black Widow sacrificed herself for Clint Barton to get the stone. Many of us held on to hope that Black Widow would return but co-director Joe Russo made it a point to state that the Soul Stone sacrifice is irreversible. This means that present-day Gamora and Black Widow are gone forever.
Thankfully, we will still be seeing more of Black Widow and Gamora going forward, just not in the form or fashion that we would most like. Scarlett Johansson is all set to reprise her role for a Black Widow prequel movie. Gamora's fate is a bit shadier as we don't know if the MCU will decide to do something to get the old Gamora back. If you saw Endgame you should know that Gamora from the past is now in the future but she is definitely not the same Gamora. It would be very interesting to see how Guardians Of The Galaxy Vol. 3 deals with "Gamora problem".