Despite the rumors and seemingly teasing a return, Iron Man and The Avengers star Robert Downey Jr. says he is done with Marvel.

RDJ recently appeared on the Smartless Podcast where he said "that's all done" in regards to his Marvel obligations, as Robert Downey Jr. finished his Marvel contract with The Avengers: Endgame.
I suppose we could argue Robert Downey Jr. could always sign a new deal, as rumors offer he may appear in some sort of cameo in Black Widow (though rumored to be a deleted Captain America: Civil War scene, which, if true, would be a part of his Marvel contract that ended), and rumors offer he might be a part of a Disney Plus series for Iron Heart, which would see Robert Downey Jr. provide the voice of Iron Heart's AI.

It's also unknown if Robert Downey Jr. is involved with the upcoming What If? animated series coming to Disney Plus, but again, if that happens, that might have been a part of his previous Marvel deal and he may have already recorded his scenes.

So it looks like, as of now, Robert Downey Jr. is no longer involved with Marvel.