It wasn't a universal default, some games like Quake and Chasm did not have reversed Y by default while others like Duke3D and SkyNET had reversed Y by default. Though all of them had an option to reverse Y, i can't think of an FPS that did not have the option.
And TBH i'm not sure how much defaults played a role back then, they all pretty much had very weird defaults :-P.
And TBH i'm not sure how much defaults played a role back then, they all pretty much had very weird defaults :-P.