TL;DR: Why does M2 not support other db types other than mysql?
This is not a philosophical question or an opinion based one. I'm asking why does M2 not support other db engine from the technical point of view.
What are the specific pieces of code that are only mysql compatible and that would require too much work to make it db agnostic?
I don't need a full list, but a few examples like "In this class / file insert class name and/or github link this piece of code is used insert code here that only works on mysql".