In theory, open world is used to designate videogames that put not restrictions on how the world is navigated or explored. In practiced however, is is employed for videogames in either the RPG or shooter tradition, and because of this fact, many people would include old RPGs in that category.
However that doesn't make any sense, people who play Open Worlds nowadays expect:
- Loads of optional content (side quests), main quest can be ignored and the game is still entertaining.
- Independent content (you can solve quests in any order and without any attachments in the future).
- Visit location once, forget immediately. Locations are like levels you beat and move on.
However none of that makes sense for old RPGs.
- There is barely any optional content.
- You can visit any location any time, but the main quest is still lineal.
- Usually you need to go back and forth between several locations, not "visit and forget".
Using open world to designate old RPGs is anachronistic, don't do this.