Okay. This does circle back to one of the other things ("plausible"), so it's a bit difficult to work with, but it's something, so I won't complain too much.
Plausibility is deeply subjective, as I understand the term. Things I might consider implausible, you might consider plausible and vice-versa. This is especially true if we refer to it as "broadly plausible", meaning we aren't requiring a narrow lens here. E.g., Dictionary.com gives "plausible" the primary definition of "having an appearance of truth or reason; seemingly worthy of approval or acceptance; credible; believable"--tying it to entirely subjective things like appearances, worthiness, approval, believable, etc. The second definition definitely isn't something you want to bring in so I won't touch that.
But there's a second component here, which is...more or less what I've referenced several times now as "DM effort required", namely, "the fantasy physics and assumptions of the campaign world (setting)", as you put it. What things are part of the fantasy physics? Well...whatever you're told are part of it, which means...anything the DM decides to include. So the plausible cannot really limit anything, unless the DM is just bad at getting you to think they know what they're doing or talking about. Almost anything can become plausible with the right preamble. That's why the second definition I alluded to above isn't something you want to invoke, as it notes that a "plausible" argument, for example, is implicitly an argument with only the superficial appearance of truth and nothing underneath, sort of a damning with faint praise situation (as in, if it were more than merely plausible, one would expect to use the stronger word instead.)
"Realism", if it does in fact find its root in plausibility, establishment of patterns of inputs and outputs ("fantasy physics"), and "assumptions of the...setting", is...thus not really that much of a limiter. Yes, it will limit some things, some of the time; I'm not trying to say it's literally 100% pure anything goes. But it's also a pretty weak limit. That's why I bring up DM effort here. Almost anything can be massaged into plausibility with enough DM effort. This becomes exponentially easier when you have grandfathered exceptions like "magic", where literally some players will get all up in arms about something until the DM says "it's magic" and then suddenly every complaint (IMO pretty inexplicably!) just evaporates. If magic is a part of the world's physics, it has to have rules too, just declaring something "magic" isn't enough--but D&D fans seem to accept the loosest, most ridiculously unbound, most uncodified systems of magic without a peep of criticism (one, of several, reasons why magic in D&D remains overpowered.)
It's fine. Your replies have been courteous and you have endeavored to explain, and you have recognized useful contributions I have made. I'm probably not going to read the rest of the thread, but I am happy to respond to this. There's a reply I owe Lanefan for a similar response, but that might have to wait until after I get some sleep.