All the greatest RPGs in history gave you room to roam. Fallout had you driving around in the desert and visiting other towns across a large map. Torment had many distinct areas and realities to get lost in. Arcanum seemed huge and wondrous right after crashing in the dirigible. And none of these are even considered to be "open world" RPGs.
Aren't they not considered "open world" only because the phrase wasn't used when they came up and when it gained popularity it was quickly associated with 3d action games?
Arcanum is very much open world by any reasonable definition. So are Fallouts. PS:T is not open world because your progress across different locations is explicitly driven by the plot.
How the hell are FO1/2 considered "open world"?
They're hub based.
So? You can get pretty much get wherever you want pretty much whenever you want. That's the definition of open world.
Nearly every game that's not completely linear is open world by this definition.
Most games are not open world.
PS:T is not open world because most of the plot involves getting places (or getting out). You figure out how to get out of mortuary, then into Pharod's place, then out of Dead nations, then out of Hive, then to ravel, etc.
Deus Ex (any) is not open world, you move between discrete locations at the pace of the plot instead of traveling freely.
Most FPS games are not open world.
DOS2 is not open world - get out of Joy, then off the island, etc.
Fallouts, any TES, BG1, Arcanum, etc. are open world (BG1 is not pure as there are some areas that are off limit based on a more or less flimsy premise, but vast majority of the world is freely accessible at any moment).
The gray area are games where you can technically roam unrestricted but geography or topology makes it easy to gate off the content or determine how exactly you will progress - say Wizardry 8.