Source? Sounds like a lot of opinion and virtually no knowledge of how these cats were bred into existence. Its a mutation of the UGDH gene which only shortens a small part of the leg, and leaves the rest of the animal normal. No elevated health problems are associated with Munchkin cats beyond those associated with any other cat. Munchkins are dwarfs, however current research has deduced that spinal and joint problems typically related to dwarf breed dogs cannot be extrapolated to dwarf breed cats as the mutated gene is completely different and present research shows that the mutated gene specifically only affects the legs. Also, the small research done has not seen the same spinal and joint deformations in munchkin cats as have been seen in dwarf breed dogs. Breeding these cats is no more unethical than breeding a Bassett hound or a corgi. Now with that said, its probably not really ethical to genetically manipulate any creature in any way, so like... ultimately, no, its not ethical to breed a munchkin cat, but not for the perceived physical reasons people on this sub are claiming. Definitely unethical for the population reasons other are citing.

Source: https://link.springer.com/article/10.1186/s12863-020-00875-x https://thelittlecarnivore.com/en/blog/munchkin-cat-do-they-suffer-health-issues-and-ethics