I believe Stan has a lifetime contract that pays him a nice stipend if I’m not mistaken.
But if he had retained ownership of all the characters he created in whole or part, he’d be as rich as Bill Gates.
Actually Jack Kirby (who created Captain America with Joe Simon, if I recall) did successfully get some of his creator rights reinstated.
It was called “work for hire” but there was a provision within the law that the creators could still reclaim ownership after a period of time (50 years?) after which I believe the characters were sold back.
I remember some of the ongoing arguments. I remember that Jack did eventually get some of his artwork back. I don’t recall the particulars of the different artists/writers who sought a stake of the money in the 1990s.