https://www.cnet.com/news/avengers-endgame-postcredits-scene-was-never-planned/

"It was never in the cards," said Joe. "It was called 'Endgame' for a reason. This is it. This is the ending. The book is closed on that chapter of the Marvel universe and a new book will get written."

In closing that book, they chose to include what Anthony called "an echo of the past" -- the clanging sound of Tony Stark (Robert Downey Jr.) building his first Iron Man armor from the 2008 movie that kicked off the MCU.