This is a long read if you actually care to see why fans of the original Halo series are so upset about it to this day.

It WAS decided but certain people like to rewrite history and say that it wasn't. Not saying that's what you're doing, but people have done this a lot and have framed it this way and that's probably why you think they never made a clear decision when that couldn't be further from the truth. 343 Guilty Spark clarifies multiple times throughout the series that humanity is Forerunner.

He eventually makes comments in Halo 1 directly saying "I am grateful to see that some of them survived to reproduce." "You can't imagine how exciting this is! To have a record of all of our lost time! Human history, is it? Fascinating." In Halo 2's original storyboard which was cut due to the infamous development of that game, Guilty Spark would lead the Arbiter to a Forerunner tomb that revealed a human skeleton. In Halo 3, 343 GS directly says "You are Forerunner! But this is my ring!"

Doing just a bit of research about the retcons of Halo and seeing what previous devs have said about the story makes it crystal clear. There's also a novel called Contact Harvest, which was released after Halo 3, it is a story about Sgt. Johnson when he was on the very first planet that was fought over during the Human-Covenant war.

In this story, written by Joe Staten who is the writer for the Halo trilogy, it is directly confirmed that humans are of Forerunner descent, and this is the reason why the Prophets want the Covenant to exterminate humanity. They had the belief, due to their misunderstanding of a Forerunner AI named Mendicant Bias, that THEY were the reclaimers.

When they discover a planet full of Forerunner markers and the AI explains that the prophets have consistently misunderstood the AI's message, it is also revealed that all humans are Forerunner. So the prophets have the covenant believe that humanity is made of Forerunners who were left behind, not worthy of the great journey, continuing their lie to maintain power and influence.

Joe Staten was not with the Bungie crew when he was writing this book. Halo 3 terminals had to have been written mostly without his knowledge, as Joe's writing has a cohesive narrative from start to finish and it doesn't make sense for the forerunners and ancient humanity to be two different things with his story explaining the origin of the human-covenant war. It is only when the terminals are introduced that the story isn't so clear-cut anymore.

To ignore all of this evidence in favor of terminals that were an afterthought compared to the rest of what the series had built up to that point is just ignorant. I don't have a problem with retcons, I have a problem with retcons that add nothing to the story. Precursors fill the original role that the Forerunners did in the original trilogy but worse because ancient humanity existing alongside the Forerunners narratively makes no sense. To this day, the writers for Halo novels try to reconcile how humanity and Forerunner are related without being direct descendants and at the moment they're basically cousins when this was never intended to be the case.