Spoilers ahead. It is a time of mourning in the MCU. They promised us death in Avengers: Infinity War, and they delivered. After years of anticipation, Thanos finally made his move on the Avengers, and, naturally, he brought death with him. It's not an exaggeration to say that half the Avengers did not make it out of Infinity War alive, but it's also true to say that not all of the deaths in Infinity War are permanent. Now, as fans wait for Avengers 4, it's up to us to determine which Avengers are temporarily dead and which Avengers are really dead after Infinity War.
Thanos' plan from the beginning is clear: he wants to kill half of the inhabitants of every planet in the universe. He views mass genocide as mercy — an act that will keep populations from decimating their own planet's resources and rendering themselves extinct. It's a mission he's been on for years, as seen by his slaughter of innocents on Gamora's home planet when she was just a young girl. But, with all six Infinity Stones, he wouldn't have to go killing people planet by planet. With the snap of his fingers, half of the universe's population will disappear. And by the end of the movie, he is able to do just that.
The last few minutes of Infinity War see Bucky, Black Panther, Spider-Man, and more Avengers dissolve into dust, but it's pretty much assumed that all of the heroes erased by the Infinity Stones in the end of the film will return in Avengers 4. (Half of them have sequels, so being alive is kind of a necessity.) However, not all Avengers who died in Infinity War did so at the hands of Thanos' magic Infinity Stones, and it is these Avengers who are probably dead for good.
Asgard's first line of defense, Heimdall, was the very first casualty of Avengers: Infinity War. Already injured by Thanos' attack on the Asgard refugee ship from the end of Thor: Ragnarok, Heimdall used his last breath to save the Hulk by sending him down to Earth via Bifrost, before he was given a final blow by Thanos and his goons.
Loki was next to die at Thanos' hands — literally, Thanos strangled him and snapped his neck for good measure. Loki's death came after a final moment of redemption in which he traded the Tesseract for Thor's life, and then tried to kill Thanos once and for all. Unfortunately for Loki fans, the MCU favorite seemed pretty dead at the end of the scene. Thanos himself promised "No resurrections this time."
Gamora's death was arguably the most enraging of Avengers: Infinity War. As one of only a handful of female MCU leads, Gamora was a key member not only of the Guardians of the Galaxy, but also of the female superheroes on screen. Thanos sacrificed her in exchange for the Soul Stone, the cost was the soul of one you loved, and the only thing Thanos ever loved, it seems, was Gamora. He pushed her off a cliff on the planet Vormir and woke up with the stone in his possession.
Gamora's dead body was shown splattered on the ground below, which definitely makes it look like Gamora is really dead for good. That said, some die hard fans have some reason for hope. Her death was directly tied to the Soul Stone, and if the deaths at the end of Infinity War can be reversed, then maybe hers could be to. Granted, that might just be wishful thinking. One thing that is for sure is that this won't be the last we see of Gamora. It's likely that she'll return in Avengers 4, if not alive then in flashbacks, as actor Zoe Saldana revealed in an interview with The LA Times that she had read the script for the coming film, which would be odd if she's not in it.
Like Gamora, the permanency of Vision's death is debatable. He had the unique honor of dying twice in the span of 10 minutes, once at the hands of Wanda, who destroyed the Mind Stone before Thanos could get it, and then again when Thanos used the Time Stone to turn back time and make him alive again so that he could pull the Mind Stone right out of his forehead, thus killing him. Vision's body turned grey immediately, and he definitely looked dead, but because his death involved time travel, rules don't really apply.
If the Avengers end up having time travel on their side in Avengers 4, who knows who could be brought back to life. But, for now, it looks like fans should be ready to say goodbye to Heimdall, Loki, Gamora, and Vision for good. RIP Avengers.