At the end of season 1 of Stranger Things, it is not clear whether the gates to upside-down are still open. Since the monster was killed, does the upside-down still exist? Are the gates still open?
We obviously have this gate to another dimension, which is still very much open in the town of Hawkins. And a lot of questions there in terms of, if the Monster is dead, was it a singular monster? What else could be out there?
We really don’t go in there much until they go in to find Will at the end. So we’ve opened up this doorway, and to us it’s exciting to talk about, like, what else is behind there? There’s a lot more mystery there to be solved.
In Stranger Things, we only see one permanent gate inside US Department of Energy-backed Hawkins National Laboratory. Which Hopper and Joyce used to catch Will. But we never see how they came out and did they even closed that gate or not.
Eleven just vaporized the Demogorgon into ashes and then disappeared with it and we only know her capable of opening or closing those doors, so it's safe to assume it's still open but we are not sure about it and it's pure speculation on my side. And even Hopper left food in a box which was supposedly for Eleven.
The first episode of Stranger Things 2, MADMAX, is set approximately 11 months after the end of Stranger Things. In it we are shown that the original portal (or "the Gate") is still open. Further,
it is growing due to the influence of a newly identified creature eventually dubbed "the Mindflayer". Workers at the Department of Energy building regularly use a flamethrower to limit the spread of the Gate.