Justice League: 8 Theories On How Superman Will Return
He's only mostly dead.
Whether you agree with the logic of making Superman such a big part of the Justice League marketing or not, it's fairly well established that he's going to be making his return at some point during the movie's events. He's being set up as the key to stopping Darkseid's invasion - as Bruce Wayne says "the world needs Superman" - and even if the rumours of him coming back different are true, we're also being told to expect to see the most traditional image of Superman ever eventually.
In order words, the Big Blue Boy Scout is in for a second coming, just like his religious parallel.
Despite Warner Bros' somewhat notorious fondness for spoiling key plot details (at least in Batman v Superman and Suicide Squad, anyway), they've done fairly well to protect the narrative of Justice League. The marketing campaign has mostly been focused on building mood and tone (well, it would be when those two things are Snyder's two biggest issues for critics), and so far, it's unclear how Superman's return will even fit into the story.
For now, the question of HOW he comes back is still the biggest Justice League mystery of all. And there are lots of options...