This the speculative hypertemporal hypothesis, that posits an alternative temporal direction, that would somehow be angular to past-present-future of the entropic block universe. Nothing of the sort has ever been observed or experimentally suggested or demonstrated. It's a purely philosophical contention at this point in time.

Everett's argument was simply that the wave function is a real feature of the natural world, and that alternative states of a given amplitude ought also exist somewhere. There are, in turn, multiple interpretations of this interpretation.

It might also be that alternative amplitudes exist in other worlds of some inflatory multiverse beyond the cosmic horizon - in which case, the evolution of the wavefunction remains purely deterministic within our own block universe. Thus, free will and human decision-making need not apply. Everett himself was determinist in his thinking, and the beauty of his interpretation is that it's entirely consistent with observation.