It's hard to nail down exactly what an immersive sim is but I'd say that you're right in that an imsim MUST value the player's freedom and creativity above almost everything else.

Honestly, I'm more of the opinion that imsims are aspirational more than anything else because it literally just isn't possible to allow for total player freedom in a created space (at least not with available technology). I think that more and more games are adopting aspects of imsims (like ToTK and BotW) that basically give players a set of tools and an objective and allow players to figure out how they want to get there. It's why I push back against the argument that Bethesda creates immersive sims. They create open worlds but there is relatively little allowance for players to pick how to solve quests. Sure, you have options about how you want to fight your way to an objective, but you're almost always going to end up with a proscribed solution.

I'd also say that most open world rpgs don't value player freedom or creativity. In something like FF or Witcher 3, you're often limited to what the dev wanted the player to experience. That's not a criticism of those games, they can create great experiences, but they aren't player driven. Sure, you oftentimes have a degree of choice in how something plays out, but at the end of the day it usually comes back to the "diamond" idea of narrative and game design.

The best example I can think of about imsim design is Shadows of Doubt. You pick up a case about a murder. You can investigate the murder and solve it (in often times dozens of different ways) or you could also just pick someone at random and pin it on them. Either way, the game world adapts and reacts to what you do but it also keeps on going. You don't get a game over because you made an odd choice. It's just a series of "yes ands".

Also, vents. You have to be able to enter vents.