In Avengers: Endgame we see that:
Loki steals the Tesseract after Tony fails to take it.
Can this action alter the present in the current universe, where they are supposedly dead? What else could be the reason that the scene is included in the movie?
I've said this in a few answers now but time travel appears to work in that the characters are not altering their own past but their future... in the past. As such any events that happen in the future past don't affect what had happened in the past past.
That said as The Ancient One explained a way to create a different timeline is to remove an Infinity Stone from its own timeline. As Loki now has the Space Stone it could be possible that he could create a new timeline and survive.
It's worth noting though that Loki is getting a spin off TV series sometime and so this could all just be a set up for that in either an alternate timeline or something similar.
Disney’s new streaming video service needs some heroes, but it’s turning to some former Marvel villains who have been drawn to the light.
Tom Hiddleston’s Loki and Elizabeth Olsen’s Scarlet Witch could each get their own limited series on the platform, which is expected to launch in late 2019, sources confirm to EW.
Entertainment Weekly, Disney's new streaming service to feature Loki and Scarlet Witch shows
Have to copy my own answer from Movies & TV Stackexchange
Christopher Markus answered it to Fandango:
Fandango: The Snap did bring back a lot of our favorite characters. Loki, is he kicking around somewhere? And what about Vision?
Christopher Markus: No, I mean we only brought back the people who were effectively disintegrated by the Snap at the end of Infinity War. Anybody who died over the course of the movie through neck-snapping or stabbing or being thrown off a cliff or having a Mind Stone torn out of their head stayed dead.
But Russo and Anthony contradict it, from Business Insider:
Clark: At the end, when Cap brings the stones back in time, does he correct all the timelines that got screwed up? Is there a past Loki still out there?
Joe: The intent was that he was going to correct the past timelines at the point that the stones left.
Loki, when he teleports away with the Time Stone, would create his own timeline. It gets very complicated, but it would be impossible for [Cap] to rectify the timeline unless he found Loki. The minute that Loki does something as dramatic as take the Space Stone, he creates a branched reality.
Anthony: We're dealing with this idea of multiverses and branched realities, so there are many realities.
But I will probably go with Russo as Loki is going to appear in his own TV series soon on Disney+ but we need to wait and watch.
The Loki variant who snatched the Tesseract and escaped from the Avengers’ custody is alive and is the protagonist of the TV series Loki (2021). However, this does not affect the fate of the original Loki, who was killed by Thanos in the main timeline.
Instead, the Tesseract-stealing Loki unwittingly created a divergent branch in the “sacred timeline”, which is the timeline that the Time Variance Authority (TVA) protects and preserves. The TVA arrested him and recruited him to work with them to fix the anomalies in the sacred timeline. His story is told in Loki.
