For those like me not willing to read all of this look at the very last sentence where I lost track ^.^
I haven't really seen it here but I've seen people questioning why sans is the only one who bleeds in undertale but I reckon I know why, and no I'm not a ketchup in pocket believer. In deltarune Susie says "Everybody bleeds, right?" Right before Lancer runs into the castle; so based off that evidence, wouldn't that mean monsters in the deltarune universe/timeline bleed. More evidence that kind of supports this is that monsters actually have graves in deltarune unlike in undertale where their dust gets spread on their favourite object. Some evidence that contradicts though is the kid who asks Kris if it hurts to be made of blood. But disregarding that the most reasonable conclusion is that Sans undertale isn't Sans undertale at all but in-fact sans deltarune :0.
More evidence that supports this comes from undertale. The machine in Sans' and Papyrus' house most believe that this is some sort of time or dimensional travel device which is pretty believable along with the picture in the same room of three characters saying don't forget. Most would speculate that thats Sans, Papyrus, and Gaster, but another possibility is that it could be Kris, Susie, and Noelle because in the beginning if you make one of your names these they are the only names described to be 'very, very, wonderful'. Along with that the shop keep says that Sans and Papyrus just showed up one day.
Something that may seem off about Sans being from deltarune is the fact that in deltarune the shop is just Grillbys scribbled out although the inside is different, but even then it wouldn't be difficult to change so maybe he is from a deltarune timeline or universe went to undertale to escape the roaring and found some way to come back to a different deltarune timeline and is trying to stop the roaring.
When Sans is a lost soul his dialogue supports even more evidence "Just give up. I did." "Why even try?" "You'll never see 'em again." This could be seen as getting out of the underground but that doesn't make enough sense he must've given up on going back to deltarune, right?
But we cant just assume all this there is the possibility that he could be from a different universe entirely. When in Sans' battle he talks about 'our' reports this could be him and Alphys or maybe someone else but who knows? he talks about the time space continuum and timelines jumping left and right and not only that but the joke books with quantum physics books inside. he most likely has at least a passable knowledge on these things so it makes sense on how he would even be from a different timeline or universe. Another bit of dialogue that stands out in his battle is "look, i gave up trying to go back a long time ago. and getting to the surface doesn't really appeal either" this could mean he gave up trying to go back to his original timeline or universe (i cant be bothered to find the difference)
anyway i've lost track so yeah hes not from undetale long story short.