I'm 99% sure Sans Undertale is Sans Deltarune.
Undertale is the 2nd game, despite being made first. Deltarune is the first world, made as a forethought to Undertale.
Sans mentions in a genocide route that he has lost the will to return, and doesn't see them getting to the surface, either.
He says EITHER. Meaning that the surface was a secondary option. What else is he hoping to return to.
"But Toby says that it's different characters with different lives."
Yeah there are different characters leading different lives, and no Undertale won't be affected. It will be made. Because Undertale is ultimately a product of the story of Deltarune.
Sans lived a different life in Deltarune. Until something happened and he ends up in Undertale. Most of my reasoning comes from 1-4 and all of Undertale with notable inclusions from chapter 5 (which I have not finished at the time of writing this out) so I'll put spoiler warning before that!
So sans is new to hometown. In fact he's just moved recently. Which is a little suspicious that the one character we know has information about things like time anomalies and such has moved to the town we the player has just arrived in shortly before us. Papyrus is nowhere to be seen and as things escalate, becomes merely a forgotten promise to hang out. But odds are papyrus isn't going to be the same age as Kris. So Sans was never asking Kris to hang out with Papyrus. He was asking us. The player.
Big stretch I know, but let's take a look at entry 17!
The writer, as we are assuming, is Gaster. Mr. Wing dings himself. And he mentions 2 others about the experiment that we have come to a conclusion is about the dark worlds based on its dark yet darker wording. Now, in Undertale Gaster has been forgotten by all of the underground, fragments of his existence slip through, but he no longer exists. And shortly before we get to the underground. (This could range from months to a couple years) the Skeleton brothers show up in Snowdin. In a house that has a machine in the basement with blueprints and a note with a hastily drawn picture of 3 friends smiling. (Keep this in mind)
Now the only skeletons there are in the underground is Sans and Papyrus. And in pretty sure Sans is at the very least 1 of these 2 other people. And wild thought, I think the other is Toby Fox.
Now I could be wrong. It could be papyrus Deltarune as well. But I just need this out, maybe ill make a video about it later.
Now spoilers for chapter 5 just a small detail.
If you are playing the normal route at least you can stop by Sans ice cream cart with Susie Kris and Noelle. He says that he has a punch card and if you draw the people on it and give it back he'll take it. Susie does a hastily done drawing of the three of them. And Sans takes it back.
And I believe that is the same note drawing we find in Undertale. It's... It's too obvious to not be the case. This isn't a story Toby has to hide because we know it already. We know how Sans ends up. Sans isn't surprised when you go to the surface like Papyrus. He knows what the sun looks like! He has a door that looks like the doors in Deltarune made for fast travel. Sans Fast travels over the entirety of Undertale! And he does this thing where he's in two places at once. So I have one last theory.
2 Sans theory. Using the information I just gave you, There's a chance that there are 2 sans in Undertale. Sans Undertale and Sans Deltarune. And that is the second person Gaster is talking to. I know its a lot in one post, ill explain better after I finish chapter 5.