What is the plural of "API" (Application programming interface)
Is it part of English irregular plurals ending in "-i"?
API could mean a lot of things. The list can be easily found on https://en.wikipedia.org/wiki/API_%28disambiguation%29.
But as it is in most cases an acronym, when you append s to it, it unambiguously indicates plurality of whatever the acronym stands for.
As in Computer science - API stands for Application Programming Interface. The plural of which is Application Programming Interfaces so the acronym in plural is APIs