1. If the appendix was both useless and deadly (before procreation), there would be a strong evolutionary pressure against it, so it likely is unobviously useful
2. Western diet possibly causes problems with the appendix - "Appendicitis is uncommon in rural Africa and Asia"
3. Anatomy involved with immune function or microbiome is hard to understand function.
From links:
The peak incidence occurs in the 15- to 24-year-old age group. However older individuals are more likely to have complications and die from the malady. Symptoms of appendicitis in older people frequently are not as clear-cut as are symptoms in younger people.
People with acute appendicitis often think they would feel better if they just could move their bowels. To encourage a bowel movement, they take a laxative. This is a serious mistake, one that may lead to rupture of the appendix.
https://www.sun-sentinel.com/news/fl-xpm-1987-04-14-87012404...
https://www.scientificamerican.com/article/what-is-the-funct...