Open world is a pretty broad category, if you are looking at the six point sliding scale of linearity vs openess, open world would start around 3, where there are no longer any levels, but a single large persistant map. This would include games as old (and even older) like the original Zelda and metroid.
Past this point it is less about the size and openess of the world, and more about the progression design determining how the player proceeds through the world. in 3 and 4's, the director has a clear path for the player to discover, the player is meant to take this path every time. Metroidvanias acheive this via a system of powerups and locks; soft locks (a ledge that is too far to reach without a powerup to jump higher, this can be overcome via other, unforseen by the director, methods, like highly skilled wall jumping) and hard locks (these are basically a disguised lock and key, an obstacle that can not be overcome without the powerup, like super missile doors). This allows sequence breaking where the player can get powerups before they are 'supposed' to.
The original Xenoblade accomplishes this via having the 'path' be story progression, and the rest of the world 'gated' by hand placed enemies of various levels, guiding most players to remember, and come back and explore later to get the great equipment that was inaccessible before (Xenoblade also has certain keys the player can optionally aquire granting access to chest laden areas). The player can break this sequence by sneaking past enemies, or learning to effectively fight high above their level, thus gaining high level equipment before they are 'supposed' too.
Level 5 and 6's arent necessarily larger worlds, or even seamless, they just have no directors intent on a 'path' the player should discover, the player can go anywhere and make progress. This is usually facilitated via level scaling.
Xenoblade X's talking point you are referring to, was that it was a seamless world, not broken up by loading screens for loading the next area. Impressive, neat, cool for immersion, but not really a first.
