Manbearcat
Legend
I think a number of folks have missed the apparently bad-faith/unfair part of Manbearcat's 1.
Preemptively using unestablished backstory or unilateral access to the offscreen
He's talking about filling in details or creating countermeasures which WEREN'T previously established as part of the worldbuilding or adventure design.
I interpreted @Manbearcat to be speaking at least mostly about doing so in the design stage. While it might not be something the PCs have encountered or had any opportunity to know heretofore, it's not contradicting anything they have.
I don't think he's saying mid-to-high-level spellcasters in D&D require bad-faith DMing. His experience might lead him to think so, but from prior conversations I expect him to recognize that his experience isn't everyone's.
A bit of clarification on @Manbearcat 's part might help here:
In this clause in the OP are you referring to backstory established in the design phase and consistently applied through the campaign (which nobody seems to have an issue with) or are you referring to backstory being changed on the fly in reaction to what the PC casters can do and-or are doing (which would be bad form)?
Lanefan and prabe have the right of it here. "Unestablished" here just means "latent content (GM notes/maps) which persists but has yet to be introduced into play because the PCs haven't interacted with it."
If this was a Moldvay Dungeon Crawl, it would be mapping, stocking, keying the ruin et al and "unestablished" would just be a trap or a monster or a puzzle that has yet to be encountered. Scale upwards and outwards from there (eg when you move to Expert) but its principally the same.
Now obviously there becomes issues of scope as you scale up from the dungeon. Information density and resolution becomes a problem; eg you can't detail everything with the kind of robustness and intensity in an entire hex or an entire region (etc) as you can with a dungeon.
So there, its a huge asset to develop principles (always do this, do x when y happens) and procedures (tables, resolution procedures for generating content on the fly, etc) to protect from biased extrapolation which lead to the adversarial nature of my (2) and (3). A very good example of this is Blades in the Dark's Fortune Roll.