I'll bet that you don't get a responsive answer to that - one that provides criteria that allow one to predict which myths the responder will accept or reject better than whether the myth comes out of a preferred book or another source. I suspect that you agree. The sine qua non of a faith-based belief is that it be unjustified.
Incidentally, between history and myth is legend - mixture of the two.
Yet metaphor and allegory (and simile and parable and fable) are literary forms that creation myths, for example, don't fit. In each case, these literary forms involve substitution of what is known with symbols. Symbol, metaphor, and allegory imply substitutions mutatis mutandis (this for that in 1:1 correspondence) of something symbolic for something literal and specific that the author has in mind. The bitter fox and the boy who cried wolf represent specific types of people engaging in familiar behaviors. The prodigal son also represents another classic type.
Myths are guesses at the unknown. One has to have an idea of what one is symbolizing or allegorizing to call it that. Plato's allegory of the cave is symbolic of man's plight of having only a partial view of reality, the man symbolic of mankind, the shadow on the cave representing man's limited ability to perceive reality - his filtered perceptions. The five Olympic rings are symbols representing the five continents that participate.
Likewise regarding metaphor such as "She was the apple of his eye." Apple is a symbol for something appealing and desired in the metaphor. The author knows exactly what the apple stands for, and so do we - an appealing female of some age and species.
And allegory. Gulliver's Travels is a political allegory in which fantastical fictional characters substitute for prominent historical figures like Walpole in the British politics of Swift's era, symbolized by the rope dancer Flimnap. We know what these things stand for, and they are specific, not place-holders for what is not known.
These stories are simply guesses, and erroneous ones if one accepts the scientific narrative. But what believer wants to call his sacred texts error? Symbol, metaphor, and allegory all sound better. The Mesopotamians and Vikings also guessed wrong, but we don't mind calling them wrong guesses, or myths, and we don't view them as being "deeply symbolic," nor call them allegories, which require specific referents as outlined. They're also just wrong guesses.
And where is the truth in them to which you refer? I think your definition of truth might be very different from mine. The Tower of Babel myth exists because of the truth that there exist multiple languages and that people often can't understand one another, but the story contains no truth - just another wrong guess about why these languages exist. Where's the truth in the Garden myth? It too is based in a truth - that man is mortal and must suffer and labor to survive - but sheds no light on why that is. It's just another wrong guess. And the truth underlying Job must be that bad things happen to good people. All of these observations need to be reconciled with the idea of a tri-omni deity, but the myths generated to explain them contain no truth about why these things are the case - just wrong guesses.
The flood myth is interesting, because one must ask why it's in the Bible. What truth inspired people to describe their perfect god as imperfect - as making an error, regretting it, and attempting to correct it. That they have seen floods that didn't cover all land? It doesn't seem like enough to extrapolate that fact into a punitive, global, life-sterilizing flood. I have an interesting hypothesis. Somebody found seashells or other marine fossils on the highest mountains. That might general a global flood myth. Apparently, all land was once submerged. That could only be an act of God, and as with the Garden and Tower myths, it must have been a punishment for sin, one involving nearly the entire human race.
But notice that by that reckoning, the flood story isn't metaphor or allegory - just error accounting for an observation.
