I’m fine with a definition where removing one part makes the system non-functional. There are many examples in biology.
The problem comes when people attach the claim “and they can’t evolve”. Ricahrd’s example of half a heart is perfect. If you removed the septum dividing the ventricles in the human heart this would be a serious problem, and they are a serious problem for some people. In the case of humans, this is an irreducibly complex system. So surely there can’t be any species that can survive not having that septum, right?
Wrong. Amphibians do just fine without it. In mammals, the mixing of oxygenated and de-oxygenated blood would be a serious problem which is what the septum between the ventricles prevents. Amphibians get around this by absorbing oxygen through their skin meaning all of the blood entering the ventricle is oxygenated. Reptiles have a partial septum, and it is enough to separate the two blood flows.