So I've been thinking a bit more about the need/prerogative to have an assumed Rule Zero.
It feels like we've generally uncovered at least 3 core purposes to include a Rule Zero.
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(C) to ensure consistency of rulings vis-a-vis the assumed fiction. Meaning, since the GM is keeper of the fiction, including secret backstory, there's an emphasis/importance to keep future fiction states in accordance with the previously established fiction --- especially if the previously defined fiction was generated by one individual long before play starts.
This emphasis on "consistency of fiction" seems to be a relevant application for sim, if you assume that "consistency of fiction" is a necessary precondition of "immersion". If the fiction is "inconsistent", it is ostensibly harder to drift into an immersion thought state. Players are fighting too hard mentally to reconcile "I'm immersed as my character" against "This situation doesn't make sense".
In cases of (C), it feels like Rule Zero's importance can diminish greatly in the absence/lessening of GM authored fiction prior to play and/or deprioritization of "immersion" through a sim agenda.
I'm bringing this all up in service to a final question---is Rule Zero a strictly necessary precondition for immersion? Absent Rule Zero, is there too much burden assumed to be put back on the players, in terms of authoring and making rulings, that they are unable to drift into the immersive mindset? Is the "immersion thought state" so temporary and fragile as to brook any interruption? It can't be easily toggled between authoring/ruling thought state and back?
When immersion is an apex priority, the chain of requirement for Rule Zero seems to be:
Consistency of fiction >> reduce player need to adjudicate >> reduce player need to toggle/switch out of "immersive thought state"
But if this the case, what is the GM's purpose relative to immersion? If the GM is only serving the needs of the immersion thought state (ITS) for the players, the GM themselves is necessarily not participating in that immersion thought state. The GM's role at that point is to provide an experience in which they will never participate. So it must therefore be assumed that the GM must find satisfaction in some other set of play conditions, since they cannot participate in the ITS.
Can you see where I'm going with this?