Depends on how you constrain that idea. Open worlds were a very early idea, but old computers were somewhat capacity limited in how much content you could have.
Here you had several town maps, including dual carriageways, main roads, side roads, one way streets. And you could just drive down any of them. They were all nondescript, but the amount of memory really limited what could be done.
There was also the games using the freescape engine. Driller, Darkside and Total Eclipse. These were all about as open world as you could achieve on the hardware of the time.
In terms of "open world" the definition is open to interpretation. I'd argue that text based adventures were open world too in their own way. So it really depends on what features people agree makes an "open world" game as to what the first game that contains all those features was.
Could even say it goes back to the Zelda games on NES. Metroidvania games might also count. Those games all have the "you might progress in any available direction" mechanic, which IMO is the core of the open world mechanic.
There's also some games like Star tropics where the whole world was open (as in you could return to previous locations) but progress was more linear.
Would super Mario world count as open world? Not as old as the NES ones I mentioned, but I'm curious. Or say if you could go back to previous worlds in SMB3, would that be open world?
Open world RPGs were always the goal, old games tried to mask the hardware limitations by using several techniques. By the time the Witcher 3 came along open world RPGs were the most common thing, in fact at the time lots of people called the Witcher a sellout because of that, it's like if it had come up a couple years ago and had base buildiechanics, EVERYONE else was doing it.
There are LOTS of examples that pre-date TW3, I'll limit myself to a few, just because it's the ones I played. In the 90s and early 2000s I used to play Ultima Online, which is an MMO from 97 that has a vast open world. But if you want first person, Oblivion is old enough to drink.
In a way, I'd say World of Warcraft (2004 onwards) popularized that.
Here's your starting place. Here's a bunch of easy quests and monsters.
You quit the starting area. Everything feels huge and really, really fucking far away. One step in the wrong direction and you're assaulted by an enemy with a 💀 for a level. Not only that, most people would only see the loading screen once before doing an hours-long playthrough and that also increased the sense of "fucking huge world"
I think the fault lies with Ubisoft and Assassin's Creed. They really championed the idea of a bloated open world stuffed with systems that don't really interact with each other, and now AAA gaming just keeps trying to stuff more mechanics in the pile.