According to Red Skull, he was deemed unworthy of possessing an Infinity Stone, and so it dropped him on the planet Vormir to become the Stonekeeper for the Soul Stone.
Of course, Avengers: Infinity War finally addressed Red Skull’s disappearance when it hit theaters earlier this summer.
He would stay there to explain the rules of the ingot to anyone who wished to possess it — specifically that they would have to sacrifice what they love.
Hugo Weaving didn’t return for the performance, instead leaving the job to skilled impressionist Ross Marquand, but the movie very clearly explains what happened to Johann Schmidt — the movie written by the same writers (Christopher Markus and Stephen McFeely) as Captain America: The First Avenger.