Euclidean geometry (which is 2500 years old), Newtonian physics and the special theory of relativity immediately come to mind.
Finally We May Have a Path to the Fundamental Theory of Physics…and It’s Beautiful explains some concepts from our universe in simpler universes
Where Physics Meets Experience, and it's sequel Where Experience Confuses Physicists tackles questions about consciousness and quantum physics, but instead the minds split in a spatial dimension which makes it easier to grapple with.
Sometimes there's a concept that can be difficult to understand when entangle with everything else that needs to be understood about our physics.
If you isolate that concept in a simpler universe, it makes it easier to explain how the concept works.
What are such examples?
(I feel like I asked a similar question somewhere at some point, but can't find it)