I've been searching for resources covering order independent transparency, but they all cover modern OpenGL (3.0+), which I can accomplish, but I haven't found anything that explains how to implement any of these algorithms in legacy OpenGL. What are the extensions that perform any kind of order independent transparency in the absence of framebuffers and what's the first version of OpenGL that provides such possibilities?
First things first: There are no dedicated legacy OpenGL extensions for order independent transparency. Period.
However there is one technique that can be used to implement depth peeling using the fixed function pipeline. The paper can be found here: https://my.eng.utah.edu/~cs5610/handouts/order_independent_transparency.pdf