Also, if the setting is a real world one, or an imaginary prepared one, or some intersection, then authority can easily by shared.
When me and my friends played a session of Wuthering Heights, one of the PCs worked in a socialist bookshop. I decided that, in late 19th century London, that would be in Soho. Then when - due to a series of misadventures - that PC and an NPC had to carry the body of the other PC, now dead to dump it in the Thames, I just Googled up a map of London, screen-shared via Zoom, to confirm that my recollection that it wasn't very far was an accurate one.
In our Prince Valiant game, we track location within Britain on the map printed on the inside cover of Pendragon, and we track location in Europe and West Asia based on maps I photocopied from a Penguin historical atlas of the middle ages. This is all public knowledge, not GM-authority-over-backstory stuff.
In our MHRP game, when action took place in Washington, DC - a place I've never been to but some of our group members have - we again used our shared knowledge to narraet things like Nightcrawler teleporting to the top of the Capitol Dome, War Machine hanging a supervillain from the top of the Washington Monument, Ice Man freezing the lake/pond/moat at the base of the monument, hijinks involving Stark-tech orbital reentry vehicle on display at the Smithsonian, etc.
In one of our BW games that I was GMing, the PCs ended up in the Bright Desert, being abandoned there by an Elven searfarer who had rescued them from a shipwreck, but with whom they had subsequently quarelled. No map-and-key resolution was used to determine that this was where they were set ashore: we knew in general terms that the PCs had been shipwrecked off the Wild Coast, and given (i) that it was
possible that they should be sailing in the vicinity of the Bright Desert coast, and (ii) that that was where I wanted the action to go (as GM) and (iii) that at least one of the players wanted the action to go there too (the player of the sorcerer with the cursed angel feather) and (iv) that another player, as his PC, was happy to be some distance away from and hence not returning to the Elven Kingdom of Celene, then (v) I just used my GM scene-framing power to stipulate that that's where the PCs ended up! But there was no secret GM backstory about this - everyone can see the map, see the general area the PCs are sailing in, and understand the basis on which I made the stipulation that I did. (While the GM may never speak the name of their move, the players don't lose their ability to identify the what and why of it!)
Later on, the sorcerer PC wanted some allies to help him deal with some Orcs. The player, who had been doing some Googling about Greyhawk in his spare time, declared "Everyone knows that Suel tribesmen are thick as thieves in the Bright Desert!" and declared his Circles check. There is no basis here for me to contest his conception of the fiction - my job is just to adjudicate the check. Given that the character has the Outcast setting as one of his Lifepaths, and furthermore we know that that involved spending time in the Abor-Alz just north of the Bright Desert, the check was clearly a permissible one. So I set the appropriate difficulty and he rolled the dice. As it turned out he failed, and so the the leader of the tribesmen who he met turned out to be an old nemesis, rather than a prospective ally. And things went on from there.
I'm setting out these examples to show how I think that shared authority over backstory/setting is perfectly workable.
I agree with all this, both as a description of
@EzekielRaiden's play and as an account of an approach to setting in a "no/low myth" context. As you can see from the first half of this post, I think the same basic points can apply to a pre-established setting too.
