An appendectomy was the surgical removal of the vermiform appendix. It was not considered a major surgery.
Elias Sandoval underwent an appendectomy some time prior to 2264. The appendix later grew back when Sandoval was exposed to the Omicron spores. (TOS: "This Side of Paradise")
In 2372, The Doctor mentioned that he had performed three appendectomies since his activation the previous year. (VOY: "Lifesigns")
In 2377, B'Elanna Torres compared performing gene re-sequencing on her unborn daughter (which would lead to the non-development of, among other things, a third lung) to a prophylactic appendectomy. (VOY: "Lineage")