Season 2.5 was intended to be an experiment pushing the limits of what could be done with a mostly vanilla experience.
This mini season disabled the use of all portals in favor of experimenting with Immersive Portals. Rather than being able to access the nether and end normally, all three dimensions were stacked vertically, with the nether forming the bottom of the world, the overworld forming the middle, and the end island forming the top third.
Accessing the nether and end both had requirements that must be met prior to exploring the associated dimension. Each of these were signified by advancements.
In spite of its short runtime and limited capabilities, Season 2.5 is well regarded by some in the community as one of the best and most successful seasons that we've had.
While Immersive Portals was the main gimmick of this season, this mini season also included an extra secret area.
The Backrooms could be accessed by breaking reality. This included methods like dying outside the world border, falling from an extreme height, and suffocating in blocks.
The Backrooms were inhabited by an Enderman known as The Entity who would chase players relentlessly. Players were rewarded based on the time they remained alive and a scoreboard with the top times was kept at spawn. Players could increase their chances of survival by finding and looting hidden chests spread throughout the map.
The experience was enhanced by Immersive Portals and created a nearly seamless wrapping transition that allowed the backrooms to feel like they went on forever.
Immersive Portals ended up being a very large headache during the short life of the season. Between interfering with entities being visible when changing dimensions and being able to get out of bounds in our secret area, Immersive Portals ended up being a larger problem than it was worth.