The fate of Tony Stark in Avengers: Endgame was planned long before cameras started rolling, as the Russo brothers have revealed that they learned about the death of Iron Man three years before filming began.
Avengers: Endgame ends with an alliance of almost every superhero in the MCU and their allies facing Thanos and his army. Thanos almost manages to seize victory with the Infinity Stones, but Tony Stark manages to use them first to destroy Thanos and his forces, Iron Man performs the ultimate sacrifice for those he loves.
The death of Iron Man and the exit of Robert Downey Jr. from the MCU was not a recent decision. Anthony and Joe Russo recently participated in a Reddit AMA, where they answered questions about Avengers: Endgame, the MCU, and other projects that they will be working on in the future. The Russo brothers were asked how long ago they knew when Tony Stark was meant to die in Avengers: Endgame and the response from Joe Russo was "We knew for about three years before we shot the film". The Russo brothers were also asked about the most difficult day of shooting on Avengers: Endgame and their response was "Tony Stark's death."
there were last-minute changes to the schedule.
The death of Tony Stark was one of the most important moments in Avengers: Endgame, the departure of Robert Downey Jr. was not a decision that was made likely, nor with malice, and the directors who were responsible for handling his ing were given as much time as possible to prepare and do it right.
Source: Reddit
