The Duffer brothers have revealed that they wrote a 25-page mythology document for Netflix back in Season 1, which delves into the lore of the Upside Down. Expect to get some big reveals about the mysterious parallel dimension in the final season.
Unfortunately, an official release date for the final season has not been announced yet. However, the Duffer brothers have stated that they are hard at work on the scripts, and production is underway and expected to release in 2025.