The actor was profiled in a new Vanity Fair piece, and they interviewed Marvel chief Kevin Feige to talk RDJ's time as Iron Man ... including what led up to it, and how it ended -- namely, with his character being killed off in 'Endgame.'

It was a huge moment for the Marvel franchise ... and there's been a lot of speculation Feige and co. would attempt to revive Downey's character in future installments, via alternate universes, etc. Welp, that ain't happening -- 'cause KF says Tony Stark's forever gone.

Of RDJ's departure, Feige explains ... "We are going to keep that moment and not touch that moment again. We all worked very hard for many years to get to that, and we would never want to magically undo it in any way."

'Endgame' director Joe Russo confirmed Downey wouldn't don the suit again ... adding, "We’d already said tearful goodbyes on the last day of shooting. Everybody had moved on emotionally. We promised him it would be the last time we made him do it -- ever."

The way the filmmakers describe it, it's almost like Downey is over Marvel -- which makes sense ... dude's been in the franchise since 2008, when he kicked things off in "Iron Man."

Speaking of that ... Feige says the studio heads at the time were reluctant to cast RDJ at first because of his tumultuous past -- but once they saw his screen tests and audition tapes, they knew they had their man. As it turned out, they bet on the right guy for the job.