This was believed in the 20th century, but we now believe the appendix is actually useful, and is basically a fail-safe in case the intestinal flora are wiped out; some will survive in the appendix and repopulate the intestine.
> gut fauna
May I be excused? My macrophages are full of slime molds!
It's called "gastrointestinal illness". From the article I linked:
"Research in 2012 reported that individuals without an appendix were twice as likely to have a recurrence of Clostridioides difficile colitis. The appendix, therefore, may act as a reservoir for beneficial bacteria. This reservoir could repopulate the gut flora following a bout of gastrointestinal illness."