The Upside Down has been a huge part of the show from the beginning, when Will is trapped there, setting off a series of events that lead to the release of the monstrous Demagorgon and a variety of creatures like it throughout the following seasons.
The show will answer the question of what the Upside Down is in its final season, but it's not certain at the moment. The Duffer brothers, Matt and Ross, are the creators and showrunners of the show, which tells the story of a group of kids growing up in the fictional town of Hawkins, Indiana, where they encounter an alternate reality called the Upside Down, which wreaks havoc on their world. In an interview with Variety, the Duffer Brothers said, "A lot of those answers for The Upside Down is really what the basis of season 5 is about."
The pressure is on to deliver a satisfying finale that will honor the legacy of the series, rather than closing it out with no answers. The Duffers said that the Upside Down is frozen in the year 1983, which is a huge part of season 5. The final episode of the season is two hours and 19 minutes and is the longest in the series to date. The person who opened up the portal to the Upside Down is said to be Eleven, a girl with telekinetic powers. She eventually escapes and closes the portal, but it has been continually opened up, leading to the release of the season 4 villain Vecna.
It would feel disingenuous to not pay off The Upside Down with the final season of the show.
Many questions remain about the Upside Down, including its origins and place in the world. The Duffer Brothers said that they were still working on the story for the fifth season of the show and that it may or may not feature longer episodes like the current season. The final two episodes of Stranger Things are set to premiere on July 1st. There are new details about the significance of the Upside Down in the fifth season of Stranger Things.
It's operated mostly as a mystery with little explanation, so unraveling that mystery should be the focus. The Duffer Brothers seem to be focused on doing that with the fifth season of Stranger Things. Various aspects of its existence have been teased, but no one knows what it is. Leaving it ambiguous would put fans off and provide a stronger narrative choice.