My own Deltarune theory is that Undertale is basically Post-Scratch Deltarune. (I mean, we get vague hints of another world in UT, so I think it makes sense.) It could be the other way around, but I like DR->UT because it means they have the chancd of a happy ending.

ut/dr doesn’t really have an established method by which a scratch-level event could happen… certainly branching timelines are within undertale’s established physics, but i don’t think it’s really worth speculating so long as we still don’t know what the actual difference is between the two timelines