There were certainly strong statements in the Watchtower against organ transplants, but I don’t know any concrete statement that it was a disfellowshipping offence. I’ve also been looking out for examples for over 20 years and I have not yet come across a story of a JW who died for refusing an organ transplant because of the Watchtower’s opposition to transplants. If anybody knows of any it would be useful to share details, including a source, if possible.
I suspect that they changed their stance on organ transplants early enough so that they had changed their position by the time that organ transplants had become commonplace. It’s worth bearing in mind that other religious groups were also skeptical about the ethics of transplants when it was a new medical treatment.
Having said all that, I do know a JW who refused a heart transplant in the 1990s who probably died earlier than he otherwise would have because of that decision. His reasoning was that he didn’t believe it was possible to get a heart transplant without blood because the heart is full of blood. So for him, and it was his own reasoning, not anyone else, refusing blood necessitated also refusing a heart transplant in particular.