Is a ruptured appendix always fatal?

Unless treated: Unless treated by immediate surgical interventions and antibiotics afterwards, it is.

Almost never: The actual death rate for ruptured appendicitis is 0.6%. Extremely rare. Non-perforated appendicitis carries a 0.1% death rate. The use of antibiotics and advanced surgical procedures has definitely decreased the death rate from appendicitis.

No, though it can be: When the appendix ruptures, bacteria from the intestinal tract can enter the abdominal cavity, causing severe inflammation and the spread of bacteria into the bloodstream. Sometimes, the hole in the appendix is "band-aided" closed by an adjacent organ, limiting the spread of infection. In rare cases, the body can heal this without medical intervention. Since we cannot predict this, we rx all.

No. : At least in developed countries the numbers are much smaller than a hundred years ago. Still, it can happen in older patients where other diseases may hide the diagnosis from doctors until it's too late.