If Marxism is a science, then it must be testable and falsifiable. At least, that's my understanding of the scientific process. It's why Freud for example is considered unscientific. This theories have a lot of wiggle room, where any result can be turned into a proof or explained away.
Isn't this what we do with the various leftist projects? I don't know, I'm just confused why it is called a science and not, like, philosophy.
Marxism is unfalsifiable, in a way, because it is self-critical. Like all science, disproving aspects of Marxism doesn't disprove Marxism in its entirety, but leads to a broadening of its theoretical framework. For example, Newtonian physics was "falsified" by Einstein's theory of relativity, this doesn't mean we throw out all physics, or even that Newtonian physics is now useless, it means that physics has developed to better explain the material world (which will happen again when physicists figure out wtf gravity is).
Someone mentioned that a classless capitalist society (anarcho-capitalism) would disprove Marxism, which is true, just as if I dropped an object and it floated up toward the ceiling it would disprove Newtonian physics. It's in turn impossible to prove that neither of these things will happen, which is why gravity and Marxism are both technically theories. But drop an object 10,000 times and it will always fall to the floor, and get 10,000 libertarians to establish their perfect utopia and it will always get overrun by bears, or something of the like.
In short, Marxism is unfalsifiable because it is not false.