[redwall_hp]

I definitely remember hearing the phrase "killer app" plenty of times in the early 2000s, and Macworld definitely shortened "application" to "app" frequently in the late 90s and early 2000s.

[weinzierl]

I heard app the first time in connection with "killer app", before I only remember the term "program". If my memory serves me right I heard "killer app" the first time in the late 90s

[arantius]

https://books.google.com/ngrams/graph?content="killer+app"

With a peak in 2002 it seems.