There is straight up proof Stranger Things was planned and I will tell you how.
It follows screenwriting structure.
I can give lots and lots of examples of shows that don't. Anything that didn't know how long it was gonna be can't, really.
Usually, seasons follow it. That's the longest you get. At best, a two season plot. A good example of this is that the first 5 seasons of Supernatural were the originally intended show but then they continued on for 10 more years, and the seasons still have structure, but the whole does not.
We have expositions, rising action, mid point, cave, elixir, climax, falling action.
The biggest ones here that fall PERFECTLY and wouldn't if they were just doing +1 indefinitely whenever they got renewed, starting at any point, are cave and midpoint.
The midpoint is a shift. Things start to break down, it starts to get darker. It is after that point you begin the spiral into "what I thought I wanted doesn't actually provide what I thought it would".
That is season 3. We know it. You probably guessed it before I said anything. The first two seasons, everybody wants their things. End of season 2, for the most part, they get it. Season: it's not working.
That's why people thought it was "petering out". They were at the phase of the story where the good falls apart, a phase that exists in every story, just on a much larger scale, making it harder to see.
The cave. The cave is the worst moment. "All hope is lost". It comes right before the climax, at the end. That is season 4.
Pretty clear, again. Season 4 ends with everyone doing horribly.
Yes, season 5 is the climax. That's pretty easy to do. But there is no empty time between those structure plot points, none where they had to fill indefinite space to stall or tried to wrap too early then went back on it. It all FITS.
Season 1 is exposition. I've described it before as prologue. The things you need to know going into season 2:
Mike and El's romantic history, Jonathan and Nancy's romantic history, Will's trauma and lore, El's trauma and lore, Hopper's guilt towards El, Nancy's relationship with Steve, what Lucas was like before Max, Dustin's history of exclusion, Joyce's last marriage in contrast to Bob, etc.
Then we have rising action. In season 1 we did not learn about the upside down at all really, it was withheld from us. Now we are. We are on Will's side of things as for the first time, the monster is represented as having a goal, not just animalistic hunting. That creates urgency we didn't have before. Rising action.
We have the mid point. The characters got what they wanted: the girl, the boy, exorcism, the gate closed, the dad, the daughter, whatever. But it isn't their miracle cure. Max and Lucas' relationship is immature, Will is haunted by a childhood that was stolen from him, Joyce is still grieving Bob, Hopper has El but he's overprotective and anxious about losing her, El has Mike but it's stifling her independence because she doesn't have any friends of her own, etc.
Because they got what they wanted, not what they needed. And sometimes it wasn't in the wrong direction what they got, but it wasn't the end. It was just a stepping stone. Like Will's exorcism is obviously GOOD, but it doesn't end it.
It's the very same concept I remember realizing when I watched season 3 for the first that I had forgotten to consider: just because the gate is closed doesn't mean that world isn't still there. And if gates can be opened once, they can be opened again.
Season 4. Max is grieving and distancing herself because she and Lucas still haven't gone back and worked in that maturity, but also just because she's depressed. Mike and El never addressed the issues they had in season 3. Neither did Mike and Will. That's where it starts. That's the spiral towards the cave.
The actual cave itself is volume 2. Will is heartbroken, Mike is heartbroken, Max is comatose, Lucas and El are hit hard by that, the town is literally destroyed. From a supernatural main plot standpoint, they explicitly lost against the villain. That is the most basic straightforward default hero's journey example: that you lose the battle to the villain. Not an insult, just saying it's very clear and direct. That is the cave.
And season 5, of course, will have the elixir and climax (and falling action). Volume 1, they start to realize what it is they have to do. Volume 2, they build to doing it. Finale, they do it and experience the relief of finally having one. They build the normal lives they've always wanted.
It is often done that the final season looks like that. Because if you know you're not coming back for another season, you will know it's the climax.
But you will NOT know it's the midpoint, or the cave.
Not unless you have already planned ahead to know "season 3 is 50% in", which it is.
You only know where the middle season of your story is to place the "happens in the middle " events specifically there if you have already storyboarded and planned the whole thing to map out that you now know it will take 4-5 seasons. The average of with is 2-2.5 seasons in, aka season 3 (because that is the point at which 2 seasons have been completed, if that math didn't make sense).
In short: we know it's planned because they followed screenwriting principles to demonstrate they knew that season 3 was the MIDDLE of their story. Exactly the middle.
And this, of course, is also how you can go around predicting things correctly. First half is what they thought they needed second half is what they really want confirmed. It's fairly easy to identify what those things are/what was missing.