The following contains spoilers for Stranger Things Season 4, Volume 1, streaming now on Netflix.
Stranger Things Season 4 was definitely worth the wait. It was bigger, more ambitious and scarier than the previous seasons. Eleven embarked on a mission to regain her powers, while the Hawkins gang was busy investigating a string of horrific murders. Of course, the murders were connected to the Upside Down. So, fans got a glimpse at Eleven's past and some hints about the inner workings of the shadowy realm.
Season 3 spent almost no time in the Upside Down, but Season 4 has changed that pattern. In "The Massacre at Hawkins Lab," Steve, Nancy, Robin and Eddie all went through the "Watergate" and fought a horde of demobats. After escaping the initial attack, Nancy suggested going to her house in the Upside Down to get her cache of weapons. However, when they arrived there were no guns, and that could have some major ramifications.
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When Nancy couldn't find her guns, she did some quick investigating. By looking at her journal, she discovered that the Upside Down was stuck in the past -- November 6, 1983, to be specific. Of course, that date hardly needs an explanation. In the real world, it's Stranger Things Day, and in the series, it was the day when Will Byers went missing. Nov. 6 was also the day when Eleven first made contact with the Demogorgon and tore a gate in the fabric of reality.
While the Nov. 6 date is significant, it's weird that the Upside Down would be stuck on any date at all. As a parallel dimension; its timeline should be running, well, parallel to the real world. So, the fact that it's stuck in time is a pretty big deal and deserves an explanation.
Even before Season 4 released, many fans hypothesized that the season would introduce the concept of time travel to the Stranger Things universe. That was because of the repeated appearances of clocks in various promotional materials, and the pattern continued throughout the season -- especially with Vecna's grandfather clock. Unfortunately, though, the time-travel development never happened (at least not in Volume 1). It's also an unlikely explanation for the Upside Down being stuck because Nancy and company were able to communicate with Dustin in the real world, in real-time.
So, if the Upside Down doesn't have anything to do with time travel, there are only two other explanations. First, Eleven's immense power froze time in the Upside Down when she opened the gate to Hawkins. Or, the Upside Down isn't a parallel dimension at all. Yes, that would be a big development in Stranger Things lore... but there's support for it.
In Chapter 7, Vecna was revealed to be "001" in Dr. Brenner's program. He became a monster in 1979 when he was struck by lightning, after Eleven banished him from the real world. Since then, he's been trying to gain access to the real world to get his revenge on Eleven. However, he wasn't able to do that until Eleven opened the gate in 1983. When that happened, Vecna initiated his grand plan -- allowing the real world and the Upside Down to meld into one. So, he created a model of Hawkins as he saw it and allowed the two realms to mix through Eleven's gate.
That means that the Upside Down version of Hawkins is nothing more than a creation of Vecna's mind, as he tries to create a reality where humans and the construct of time won't hinder his power and his concept of reality. At this point, a lot of this is still in the realm of theory, but it would explain why the Upside Down is stuck in 1983.
To see why the Upside Down is frozen, watch Stranger Things on Netflix. Volume 1 is streaming now, and Volume 2 arrives on July 1.