Is the existence of a soul defeasible? Is the hypothesis that a metaphysical entity completely outside of spacetime and undetectable or observable by any known means scientifically falsifiable? Maybe it is. If it acts on observable things, then its influence is within the observable universe.
If such an entity existed, what would it predict? I would expect it to store all our memories, so when we get to Heaven we’d recognize grandma when we saw her. If the soul is the incorruptible repository of our memories, then it ought to work prior to our deaths. It would probably that which is aware of being aware within us.
It’s pretty clear there is no entity inside us that has all our memories locked in it and carries them on with it after our death. I say this because neuroscience has established specific areas of the human brain that handle specific tasks. When one of those areas is damaged by trauma, a stroke, a tumor or such; the ability it controlled is gone. These abilities can be highly specific, as in losing the ability to hear, or to remember names including your own. One area lost robs you of the ability to form short term memories, but long-term memories already stored remain intact. Lose the long-term memory area, and that is gone. You can form short-term memories, but they will disappear without leaving a trace. A stroke
If there were a metaphysical entity inside us or hovering over us with all our thoughts and memories within it, it could easily route lost abilities to other areas of the brain. But that does not happen. Only through long, arduous therapy can we learn to use a new area of the brain to pinch hit for a damaged one. And even after therapy to set up a new center for long term memory, or knowledge of how to play a musical instrument, all the memories that were in the damaged area must be relearned or they are forever gone. So it seems to me there is no rational reason to believe a soul exists.