Physics-inclined people tend to be 4-dimensionalists, so I don't think they'll object to describing wave functions in terms that account for them at all times. Even indeterminists (i.e., collapse theorists) can accept that we can talk about what will be true of electrons in the future, though we can't even in principle know some of those facts in advance.

Does the proposition "the spin will be measured up" belong to "everything that is true about the electron"?

de Broglie sez: "Yes, that belongs to everything that is true (about the electron's wave function). But at least one truth about the electron (its position at any given time) is not accounted for in the wave function. (This explains why the Schrödinger equation, although a complete description of how wave functions change, is not a complete description of how physical systems change.)"

von Neumann sez: "Yes. And the wave function encompasses all these truths. But there is no linear dynamical equation relating all the time-slices of the wave function. There are more free-floating brute facts within wave functions than we might have expected."

Everett sez: "Yes... well, sort of. The formalism for 'the spin will be measured up' is a component of a truth. But it would be more accurate and objective to say something like 'the spin will be measured up and down' (assuming it was in a prior superposition). Thus the wave function encompasses all the truths, and evolves linearly over time in accord with the Schrödinger equation. Win-win!"