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I remember reading it a few years ago and being really surprised, but now I can't find it. I probably misread back then. Mortality for general anesthesia is very low.
https://www.ncbi.nlm.nih.gov/pubmed/20215610 gives a 25/80,000 3-month mortality rate, and a slightly increased (but p=0.11, not statistically significant) mortality rate over ~5 years. But thinking about it a bit more, it could be that having only one kidney becomes a real problem when you're 70 or so, which the study wouldn't cover.