Yes.
I didn't say otherwise. I said that a sandbox game may not have a plot. That is to say, it may not have main events, as in a film or novel, forming an interrelated sequence. It may be a series of largely unconnected events with little narrative cohesion. I suspect that quite a bit of classic dungeon crawling was like this. And some contemporary OSR gaming is like this also: there are events (in the sense that play occurs), but not an interrelated sequence of main events as in a novel or film.
All plots are, by definition, linear - they are sequences of main events. (I'm putting to one side extreme avant garde novels and films. No one in this thread seems to be articulating that sort of approach to RPGing.) When the players summarise the events of the sandbox, they will fit into a linear (probably temporal) order.
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Well, they might be "plots" in the sense of "plans made in secret by a group of people to do something illegal or harmful" (the other main sense of the word offered by Google) - in this sense, a plot is near enough to the same thing as a conspiracy.
But from the point of view of the story elements of a RPG game, what you describe sounds like backstory. But it's only a plot - in the literary sense - if it is "the main events of a play, novel, film, or similar work, devised and presented by the writer as an interrelated sequence." If the PCs never interact with said backstory, it can hardly be said to constitute the main events of the RPG considered as a work similar to a novel or film. If no one at the table but the GM knows or cares about them, they're manifestly not the main events. The stuff the PCs do is what makes up the main events of a RPG campaign.
But the same point I just made to billd91 applies: if the players, and hence their PCs, ignore these "breadcrumbs" then, ipso facto, they are not elements of the plot of the game, because they are not main events. The GM may be amazed by the beauty of the backstory s/he can see, but that doesn't make it the plot of the game. Hence my remark that "In a sandbox, more or less by definition, the GM does not author the main events, nor contrive them into an interrelated sequence. To the extent that such a thing occurs at all - and it may not - it is done by the players." If the players are free to ignore the GM's backstory, then only they can bring it about that that backstory is some component of the main events of the game.
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If the "plot" is flexible in the way you describe, then it seems that ipso facto it's not a plot. It is one of several candidate plots. Until the actual sequence of main events is established, the plot isn't established.
But what you describe is still, in my view, a railroad. If the end point is already known to the GM, then however colourful and exciting the detours along the way, they are ultimately being driven by the GM, with a pre-given outcome in mind.
The idea that a game in which the GM chooses the villain, the overarching story, what the campaign is about, might not be a railroad is extremely foreign to me. I take it for granted that the players will choose the villains (ie their PCs' enemies), that what the campaign is about will be some sort of collaborative thing, and that the overarching story will be established via play. That's how I've been GMing since about 1986.