If the past still exists, presumably it would exist in the dimension of time or spacetime. Again, on these empirical questions, I’d defer, more or less, to the scientific consensus. If the passage of time is subjective and illusory, and physical time has no arrow, both the past and the future are fixed at the macro level. But relativity isn’t yet compatible with quantum indeterminacy, so the question of time is somewhat speculative and up in the air. I know Lee Smolin disagrees with the discounting of time’s arrow in physics and cosmology.

I don’t know whether it makes sense to say the past exists, but whatever happened in the past clearly fixes the present. If I buried a treasure chest in the earth years ago and no one else finds it, I could dig it up again in the present and the treasure would still be there. The finding of that treasure depends on the reality of that past event.