Function/Method names, on the other hand, should be written so as to make the most sense to the humans reading and writing the code

Of course—that’s why we have such classics as stristr(), strpbrk(), and stripos(). Pretty obvious what the differences are there.

But to your point, the ‘intuitive’ counterpart to ‘zeroth’ is the item with index zero. What we have is a mishmash of accurate and colloquial terms for the same thing.