Immersive sims require the two things in the name- immersiveness and simulation. RPG elements aren't really needed at all, though by their nature RPGs tend to have a lot of the same elements that immersive sims have as well.
While it mostly has the immediate elements of immersiveness Bioshock's attempts at simulation are basic. There's no real emergent gameplay, enemies and the environment don't behave with much realism, stealth doesn't work well (largely due to the engine used, to be fair) and, critically, it's pretty much impossible to suspend disbelief of Rapture being a Real Place existing outside of the narrow confines of being a game. That's not because it's set at the bottom of the ocean, it's because it fails in terms of verisimilitude- internal consistency/ realism such that even if you start out accepting that Rapture, plasmids, Big Daddies/ Little Sisters exist they do not make consistent internal sense except as part of a game system.
In terms of any Obsidian game being an immersive sim, who knows. FONV was probably closest of their current lot, and from Troika both Arcanum and VtMB had elements of immersive sims. I'd suspect not, immersive sims take a lot of effort for not much reward.
Also, its typically the player doing the interacting and not a player character. I mean they might have stats, skills, and upgrades, but an immersive sim lets you do whatever you want, with as little between the player and the game as possible for what it is.